By tempo
90 BPM rap beats
90 BPM is the most common tempo in rap for a reason: it sits right in the middle of comfortable speech. One bar lasts about 2.7 seconds, which is long enough to finish a thought and short enough to keep momentum.
If you are learning to freestyle, this is the tempo to start at. Most people can hold a line at 90 within a week. It is also the classic boom bap pocket - a lot of nineties hip hop lives between 85 and 95 - so what you practice here transfers straight to that catalogue.
A note on double-time: 90 BPM with hi-hats on the eighth notes sounds identical to 180 BPM to most ears. If a beat feels twice as fast as the number says, that is why. Both readings are correct, they just count differently.