You can check it with a watch or timer by counting your pedal strokes for 10 s and multiplying by 6, or 15s and multiplying by 4, etc.
Riders generally self-select a decently efficient cadence for themselves and it varies quite a bit from <90 to >100. Most are probably somewhere in the 90s.
If you want a bike computer, you can get one that ranges from 10 bucks at Kmart to a few hundred bucks. Putting them on just takes a few cable ties and the manual.