You can detect the beat by ear. Most pop music has a 4/4 time signature, so put the cursor in Audacity at a strong beat and select an area of track containing four quarter-note (crotchet) pulses (that is, an area up to the next strong beat). Note the length of the selected area in seconds as given in the Selection Toolbar at the bottom and divide it into 240.
For example, if you find the four beats take 2 seconds, 240/2 = 120 BPM. If you have a 3/4 track like a waltz, divide the length into 180.
Alternatives: This browser-based tool allows you, while playing your track, to tap any computer key to the the beat and the BPM will be displayed:
http://www.all8.com/tools/bpm.htm.
It is an embedded Javascript application so does not actually need an internet connection to run, once loaded. You could save it as an HTML document and run it offline.
Also see https://forum.audacityteam.org/t/how-to-calculate-musical-tempo-bpm/27107/1
Once you know the BPM you may wish to generate Audacity labels at each beat:
https://forum.audacityteam.org/t/beat-per-minute-labels/20785/1
Gale