1. They're holding the microphone by the head.
Also known as "cupping the mic" (a lot performers just do this because it just looks cool; so it's not a 100% dead give away.) Cupping the mic deadens the sound of the singer and should be avoided at all costs under during live performance.
To show how much cupping affects the sound, keep in mind that this "technique" is often employed by metal screamers in order to add fullness, bassiness, and intentional distortion to their screams!! This has no practical application to 99% of singers so if you see a pop singer cupping their microphone without any ill-effects... it's likely pre-recorded.
2. The microphone doesn't make noise when as it's moved around and handled.
This is ESPECIALLY true if the singer is "cupping" the microphone and you can't hear the movement of their fingers. There are also no "popping" sounds from pronouncing Ps and Ks (though, singers should be trained to avoid these pops so it's not a dead give away.)
3. It sounds perfect.
Every word is audible and there are no differences in volume between high and low notes. It sounds like the CD; almost too good to be true. It just... doesn't sound "real." There's no way of really explaining it.. there's a certain "fakeness" to it, like a teenage girl's instragram photos.
4. The singer sounds drastically different depending on the recording you listen to.
One recording sounds nearly studio "perfect" and the other sounds "raw." Generally; a well established singer isn't going to completely change their technique from song to song while singing within the same style UNLESS it's intentional. If it sounds intentional and still has the same presence as the other recordings, then it's a stylistic & artistic choice vs. lip syncing.
5. They're performing complex dance moves without losing their breath.
Sorry. It's just not possible to run around the stage doing backflips while singing without missing a beat.
6. Not moving the microphone away for high notes.
High notes are inherently louder than low notes. Singers will move the microphone away from their mouths on powerful high notes and closer on soft, intimate notes. It's pretty noticeable when done live.
Examples of lip-syncing vs. non-lipsyncing
1. Michael Jackson
Obvious Lip Sync Example. Michael jackson is dancing, running around the stage, and even SINGING WITH A MASK ON and there is not a single mistake. It sounds like someone is playing a recording of his CD. It just.. doesn't sound "real." He's cupping the microphone pretty intensely as well; sometimes going so far as wrapping his entire palm around the microphone as if he were a metal screamer!!! He doesn't really move the microphone away for high notes.
Here, the sound is audibly different!!! It sounds much more "raw" much less plastic. It's real! The volume cuts in and out, his intonation isn't always perfect; he's a human being again!!!!! He's still cupping the mic slightly.. (bad MJ!) but not to nearly the same extreme. His high notes require him to move the microphone quite a distance away!
2. Arianna Grande
However, what makes the issue a bit more complex is that a lot of singers do a mix both lip-syncing and actual singing throughout a performance. Some of the notes are real... some aren't!! I even suspect that some singers have lip syncing tracks which have intentional mistakes in them... but that's a whole different conspiracy!
Why does Arianna Grande sound so different in these two separate "live" performances?
This performance is "live" but it's appears to be mostly lip synced. You might not think so at first.. but once you compare this performance to the example below, it becomes quite obvious. The lyrics in this performance are easy to understand; they're crisp and audible. No major "blips" in her voice even in the extreme ranges (though, there are a few; she's probably mixing lipping & real singing.)
Despite cupping the mic throughout the entire performance, there are no mic-handling sounds. It also doesn't seem to affect the sound of her voice. The volume doesn't even really change. She doesn't drastically pull the microphone away as she sings higher notes. It has that "perfect" sound.
However, in this recording, 8 months later, her lower range is hard to understand; borderline inaudible at times. It's also quite breathy! It lacks the "clarity" of the first recording. Why is her technique so drastically different?
She's not cupping the mic as extremely as in the first example; and it's audible. Cupping the mic reduces clarity and emphasizes lower harmonics; potentially contributing to her muddy sounding tone.
Higher notes have noticeable volume differences. She moves the microphone away much more drastically than in the first example. She sounds like uh... like.. dare I say... a real, imperfect human being.
Almost every mainstream singer lip-syncs. If the same unrealistic expectations we place upon celebrities were placed upon you, you would too! Performing such difficult music night after night is nearly an impossible vocal feat! Many opera houses are even beginning to employ hidden microphones for that extra boost (but they'll probably never tell you this.)
That being said, it's important to have realistic expectations of your voice! Just like anything that comes from mainstream celebrities; these are images that are impossible to actually live up to! Celebrities spend a fortune on hair and makeup just to leave the house! Their photos are edited & manipulated to look PERFECT. The men are on steroids and the women are 1/4th silicone; it's unachievable because it isn't real!
Are you aspiring to be something in-human? Check out some of your favorite singers "live" performances; are they really live? What does their voice REALLY sound like? Be sure your standards are realistic!!! Aspire to be AUTHENTIC vs. perfect; you'll never be perfect and the best singers don't try to be.
Here's one of my favorite singers, Jeremy Jordan. Note how he doesn't meet a single one of the 6 give away signs mentioned above!! None of his recordings EVER sound drastically different from one another. Even his CDs sound the same!! In fact, he prides himself on the fact that in his recent musical movie all of his performances were recorded live. He's as close to perfect as a singer can get without being fake. THIS is a realistic singer to aspire towards becoming. Fuck Arianna Grande.