If you can tap along with the 'pulse' which will generally be 1/4 notes, you should be able to use a metronome, click track, sequencer, or (http://www.Dtab.underwaredesign.com) even. To reproduce the pulse and thus the number of beats per minute.
Generally in pop/rock/country music the pulse is considered the 1/4 note. The rate of these notes (per minute) ranges in the 80->120 beats per minute mark.
In other words to play, at 120 quarter notes per minute (120 beats per minute), your playing 2 quarters (beats) per second. At 80bpm, your playing (80bpm/60seconds) 1.333r quarters per second.
Each quarter can intern be subdivided again. The most common forms are binary (divisions of two, or ternary division of 3 triplets) ...
So per quarter (per beat) you can divide this space into
Common
1,2,3,4,6,8
and even,
1,2,3,4,5,6,7,8,9,10,12 easily.. (.. 11 has no nature parent..
)
it's also worth noting, that a common recording trick, is to propel a songs chorus by pushing the tempo (beats per minute) up fractionally.
Anyway. To estimate the BPM of a tune manually, you can just count 'up beats' the snare hits, or the 'down beats' bass drum pulse.
Start the tune, tap along with the snare (ignoring any embellishments) for 10 seconds. This effectively gives you the number of 1/2 notes per 10 seconds.
To calc the Beats per second..
BeatsPersecond = (Taps * 2) *6
This assumes you can hold a tempo with wavering, and the music in a common time. aka 4/4