Simple answer - tap your fingers to the rhythm of the beat on a table for a minute and count the number of times you touch the table
Techno answer - download Mixmeister BPM analyser, instal, upload track into player, and let it do the hard work for you
Edited to add - on re-reading I see you want to find tracks of a similar BPM. Run your entire music library through Mixmeister and it will detect the BPM. Open music through WMP (other media players are available) and make sure you have ticked the column to show BPM of the tracks
I have done this and created playlists for running a certain tempos 
Edited: 23/02/2012 at 13:56