If you have a digital thermometer with a probe you can get a good surface temp reading. A temp-gun would be better for that but I know that's likely overkill. If you are using an analog (dial) thermometer they can be notoriously inaccurate.

Quite a while ago we'd used a couple of those type pads - one ZooMEd and one ExoTerra i THINK. If it is the kind with the adhesive backing - don't adhere it - once you need a new tank or the pad fails (one of our pads died rather quickly after managing to crack the bottom of a glass tank) they can be a nightmare to get off. The other one (will have to check brands this evening) is still doing well heating a lizard tank.

Any time you are using a UTH with a tank it's a good idea to prop up the tank a bit so that you avoid creating a heatsink underneath and allow some air exchange.

Also on the temp maintenance - what sort of lid/covering do you have for the tank? Without one or with an all screen one the heat/humidity easily bleed out right through the top. We had to cover the screen lids with contac paper (then cut away the corners to allow air exchange) in order to keep the heat and humidity up where they needed to be. My wife still houses her liasis fuscus in this manner and it positively thrives in there.