There's a few different signs. Snakes can be affected differently. If you notice anything odd in your snakes behavior, that is something to monitor.
You may not even notice anything different at first either.... But here are some things to watch out for.
One is the snake will be very agitated. Lots of moving around, not staying still, bumping stuff over.
Another is the stargazing or loss of balance. They start rolling tier heads back in odd ways or coil the first half of thier body upside down.
Another is lethargy. They are just listless and limp.
The 2nd and 3rd are not good. Go to a vet immediately.
For the first, sometimes a simple bath with dawn dish soap to get the mite product off and a simple temporary housing with no spray will help.
Make sure to follow the instructions so that you don't risk an overdose. Mites can be scary! But don't over react and risk injury to your scaled friends