If you're DJ'ing for dancers, you should learn what ranges of Beats Per Minute (BPM) dancers like to dance to. Different styles have different ranges so you may want to change it up if you're trying to accommodate dancers with different styles. You can only do this quickly if you've organized the BPM in your existing collection and you have a sweet program like MpDJ.
MpDJ allows users to tap to the music to get an idea what should be in BPM field. To to this, just select and song and edit the song's properties. Make sure you do 8 consistent taps before you use a value from tapping to get results. This data is saved in the berkeleyDb and in the ID3 tag of the file.
If you want to do this all in the console a little bit more efficiently, see bpmcli.