BPM is only one factor when selecting songs for mixing. Other elements such as key, rhythm pattern, vocal renderings, and distinctive chordings and melodies are as important. Experience and instinct are developed through listening to and working with your music.
Some people have a natural instinct, others have to work hard to develop it.
If you know your music, you will probably have a pretty good feel for its tempo and will find that knowing the BPM is actually least important. Your ear should always be your guide when blending songs. The BPM presented in D.J. Rhythms are calculated with a stop watch. For a standard 4 beats per measure dance track, count 1,2,3 and start the watch on the 4th beat. I've been doing 50 beat counts, but recently switched to 32 or 64 beat counts as this matches the measure patterns of most songs.