How do I know if my stamps are washed out properly?

You may it more convenient to first remove the excess liquid by dabbing it onto paper in a bin, this means there is less liquid to get wash off in the sink. It is easier to wash out larger format sheets by attaching them to a clip board and brushing against the flat surface. Hold the surface of the sheet under the hot tap to remove the majority of the liquid. Then squirt a generous amount of washing up liquid (dish soap) on it and rub it in a circular motion with the brush provided until it is clean.  Then rinse and repeat the process. It should look clean without any streaks of polymer. Check it against the light before finishing.
 
Remember that stamps do not gain full hardness until after post exposure (the final hardening under water) so take care not to damage them. Normal level brushing with a soft brush will not damage the polymer. Do not leave stamps in water for longer than you have to and do not let get water get under masked tamps, otherwise they will swell.
 
If the sheet has not been washed out you would expect to see the centre’s of the letters ‘e’’ and ‘a’ filled in. there would be an over-inking in certain areas as you would lose detail. It is easy to over-wash a plate and it does it no harm, but it is catastrophic to under-wash it and the stamp sheet be rejected.