with a single tank I dont recommend a temp gun, you can get a digital thermometer at wal mart for $10. People who use temp guns mostly have multiple ball pythons and it makes more sense to buy a temp gun that a $10 thermometer for every enclosure.

Also placing a probe inside the tank can be bad, if the snake urinates or water is spilled on the probe it will tell the thermostat that its a lower temp, this will make the heating pad turn on and get too hot.

I dont want to over load you with info here though, it seems to me like you might be kinda new to this. Once you get a thermometer with a probe and a thermostat we will gladly step you through setting it up on this web site...

Bottom line is you need to get the thermometer and thermostat ASAP...