Tyra's voiceovers are usually easy to tell for a number of reasons.
1- They never actually show her talking. They occur when the focus is on someone else. They'll usually show one of the girls nodding or something.
2- The sound quality changes. It sounds like it was recorded in a studio as opposed to on a wireless mic.
3- It's not always Tyra. I swear that half of them are done by a semi-decent Tyra impersonator.