How can I determine if the photoresist film is exposed correctly for sandcarving?

I always teach people to hold their design up to a light – if you can see through the clear areas, the resist has been removed properly. If there appears to be a haze or gray areas, you may need to washout the design a little bit more. Be careful not to over-washout the design; fine details will be lost, and the resist will start to separate and break down.

—Glassmith2