You can get a tester--it looks like a positive, and is often integrated with a screen mesh counter--they're basically the same thing. You just need to make sure you're lining it up properly with the angle of the halftone to get a good reading. Most have a lower limit of 40-60, but you can usually tell if it's a multiple--i.e. 30lpi will read at 60 and 120... sort of.