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is there anything that i can buy to check the water temperature (that's not that expensive, but is worth what it costs) just to make sure that it's not too cold or hot??

digital themometers at walmart in the pet section cost less than three bucks.

will they pick at them?

It goes on the outside of the tank. It's adhesive and it sticks to the glass, or plastic in your case, and gets a reading of the water temp. Just make sure you stick it below the water level.

I've never had good luck with the ones that go outside the tank, and haven't found them to be that accurate compared with digital thermometers that have a probe that goes inside the tank under the water. I use a few different brands, the less expensive one is by ESU and sells for something like $5.99 (cheaper on-line). I have a painted that's in love with the probe and spends a lot of time fluttering at it.

I double check my sticky outside the tank one with a regular themometer and both are accurate. I think part of the key with them is not placing them on the tank right outside the heater or light sources. And the same goes with the outside of the tank. Make sure there isn't a light shining it there either.

I've noticed that my stick-on thermometer is always 2 degrees lower than what my other thermometers read. It's always good to have an extra one to double check.

I have a stick on thermometer on one end of the tank and a digital one with probe on the other side. They both always display the temperature accurately.

I use a suction cup one found at Wal-Mart as well that goes on the inside. The tube is thick enough for baby turtles' well being (don't know about larger turts). For some reason, I found this one more accurate than the sticky one. I think too many variables influence the sticky one on the outside (room temp, air vents, heat sources, etc..). I just have more confidence in this one, I am using it in the fish tank, as well as the turtle habitat. Do you guys think I should put another thermometer to measure the basking temp?

I used to use one that suctioned to the wall of the tank as well (it could have come from Walmart). I stopped using this type of thermometer when I looked at it one day and found that the bottom had fallen off and some of the contents had fallen into the tank (the balls are not mercury so at least that was good, but it was a pain to clean up).