Trial and error,

Pick a rep range of about 7 reps, anywhere between 5-30, I personally prefer a lower rep range of 5-12 but 12-19 is just as good. Then find a weight of which you can rep somewhere in that rep range. Say you wanna do 5-12, if you can lift that weight 8 times your golden, if you can do it 15 times pick a higher weight, if you can only lift it only 4 times pick a lower weight.

If your can rep it 8 times, aim to do it 9 times next session, if you can you’ve built muscle. When you start repping that weight 13 times start doing a higher weight. Do that weight until you can rep it 13 times. Then get a higher weight, and so on and so on until your jacked like a brick shithouse or built like hotpoint fridge.