Here’s a quick tutorial for new REAPER users. Often there are times when you want to figure out the tempo of a performance. The easiest way I’ve found is to separate 1 bar of the performance using the Tab to Transient and separate region functions. I then select this area, and right click in the ruler, selecting “create measure from time selection (new time signature)”
Check out the gif below (click to enlarge) and let me know if there’s anything that’s unclear.