Finding song tempo
First make sure timebase in Project Settings is set to “Time” rather than “Beats.” Split a section of audio that spans at least a measure. Use tab to scroll through transients and split where you want. Press t to set Time selection to this item length. Then right click the ruler and create measure from time selection (new time signature).1 This can be done to make a tempo map if the tempo drifts throughout the song.
The easier method is to just drop the tracks into Melodyne and have it tell me.