In order to know if the time you spend at work might be classified as overtime by the FLSA, you have to know how work time itself is defined. The FLSA language defining what constitutes work itself is vague enough, though, that each situation probably has to be figured out on a case-by-case basis.
For instance, in the 2005 US Supreme Court case of IBP Inc v Alvarez, the Court ruled that the time that employees spend walking to their production area after putting on required work gear is compensable. The time spent waiting to take the work gear off also is compensable. However, the time spent waiting to put the first piece of gear on before starting work is not compensable.
But all of this can be changed under a union or employee contract, if you’re under one, so, as always, make sure you read your employment contract carefully under any and all circumstances.