Go to Wallmart and buy an Accurite digital thermometer for $12, tons of people here swear by them. It will give you your hot side/cool side temps as well as humidity.

Also, you really cant go by the temp on the substrate because the bottom of the cage will be much hotter. You can have a 95 reading on the substrate and have the bottom of the cage read over 100 which is not good. This is where it would be nice to have an infrared temp gun to do instant spot check in your cage for peace of mind. As long as the bottom of the cage is at your desired temp you should be fine. Your snake will burrow down in the substrate if it wants more belly heat from the bottom. Never let the bottom of the tank get too hot. Your ambient temps are another story all together and this is the hard part when using glass aquarium style setups.

I will let some other people with experience in tank setups help you on this. There is a nice sticky here for tank setups which you should read.

Rule of thumb though is that you want to maintain a constant temp of around 90-95 hot side/80-85 cool side with a humidity level of about 50%, slightly more during shed cycles. If your tank is fluctuating and dipping constantly you are going to end up with a sick snake. You did the right thing coming here though!