You can tell while your doing it.
Make sure you prep right.
-Wetsand with wet 1500 grit paper
-Wipe with rubbing alcohol, don't touch lens after this
-Spray Niteshades on in light coats. 4 or 5 very light coats is a good amount. If you start to get what look like little tiny threads on the lens, you're holding the can too far away while spraying, you want to hold the Niteshades about 8 inches away which is kinda close compared to other sprays.
-Spray Clearcoat on in heavier coats. This part is kinda tough. You don't want light coats when using canned clear coat, it won't look right, you have to put it on heavy. You don't want it to run though either. So, shake the can well and start spraying. You'll see it filling in the little dimples with clear, right before it starts to run you stop spraying. I did two coats like this and it still looks great a year later.