The easiest way: lay a well-fitting pair of pants flat. Measure from the crotch seam straight down to the bottom hem. That's your inseam.
If measuring on your body, stand against a wall in flat shoes. Measure from your inner thigh (where pants crotch sits) down to the floor.
Standard inseams: short (28–30"), regular (32"), long (34"), extra long (36"). Most brands offer at least 2 of these.
If you're between inseams, size up — you can always hem shorter, but you can't add length.
For jeans specifically, account for shrinkage. Raw denim can shrink up to 1" after first wash.