The cheer segment occurs before beginning a stage performance. If you leave the performance to the idols you won't get a chance to cheer with them, which is another reason why leaving them to do it themselves isn't a great idea.

How to Distinguish Cheers

By watching the mouth, you can judge the cheer before Lives, Auditions and Festivals, 

Essentially, you should pay attention to the lip flap for long sounds.

◯ Mouth flapping    - Mouth closed

Mouth Movement Cheer Specifics
◯◯◯-◯ ターッチ Closes for a moment right near the end
◯◯-◯◯◯ トップアイドル Compared to タッチ, closed for a bit longer
◯-◯-◯◯◯◯ エイ!エイ!オー! Closes twice, each for a fairly long time
◯◯--◯◯◯◯ ファイト!オー! Seems to be building up power when she closes her mouth

You can tell the cheer from her expression, but be cautious as it is different for each girl.

In practice, their mouth movements are the same, so pay attention to, and remember, each idol's habits.