Hard wax shouldn’t leave a residue but little pieces might break off. In that case you can actually put a dot of strip wax on top it and pull it off, or apply more hard wax around it so you can get a better grip on it.
As for invisible sticky residue of strip wax, I do the front of legs, apply after cream and with my gloves on and pay attention to the feel the area as I’m doing it. You will feel anything slightly raised and you just need to put some oil on a wax strip, press it onto the residue to let it warm a little and wipe it straight off. I do this before turning the client over and doing the same on the back of legs. It saves them getting so stuck to the paper as they turn. I don’t suggest doing that on a bikini lol but you can do the same thing with the strip and oil and go by sight. Is easier to see on bikini, legs I swearI just don’t see residue sometimes a small it’s so clear..or at the end give one to the client and explain its for ‘sticky bits, hold and wipe away.’
Always seems to be a random bit somewhere like a foot or a finger and I’d rather they told me or were able to get it off than be shy and go home with it still on them and getting on their clothes.
If you mean when you rip the strip and there is loads still on the skin it might be too thick. But do it again with the same strip will remove it. Firm pressure, stretch and pull. Wax sticks better to wax than a clean strip.