The temp gauge on my car will only go up to about the 1/10th point, even after a long drive. Is there an easy way to see if it's the thermostat that's gone bad or the temperature sender?

 

well. feel your bottom rad hose when your guage gets to that 1/10th point and see how hot it is

 

It's almost certainly the sensor or gauge.
The thermostat will usually fail in one of two basic modes...either it fails to open and the car catastrophically overheats (you'd know) or it fails to close and the car takes a longer time to warm up.
If you're not pushing coolant into the overflow tank or blowing hoses (along with billowing clouds of coolant smelling smoke from the exhaust), the thermostat is probably fine.

At any rate, a faulty thermostat will not give you a lower than normal reading from the gauge.

I'd bet on the sending unit being the culprit.