Suzuki: poor cabin heat, coolant levels, suzuki sidekick

Question
One last question concerning my 95 suzuki sidekick jx 16 v 5speed.
  coolant levels are good,  i have pressure and heat in all hoses but bottom one to radiator seems cooler,  but still very low cabin heat compared to our 92 tracker which is like driving a sauna on wheels (of course the 95 that I use for paper delivery is the one with poor heat) can I check the operation of the thermo by taking the cap off radiator and watching coolant level or taking the coolant tempature to see if thermo is faulty before I buy  a new one or before i get a new heater core?  Thank you so much for your help, I've tried calling a local suzuki dealer and they wont give any good advice because its to old.

Answer
If any "stop leak" hqasw been used, it could have pasrtially plugged the heater core.  You need to verify with instrumentation the temp of the thermostat opening.  Using a thermometer is good.  factory thermostat is 80C (192F).  If you live in snow country, install a hotter than factory thermostat.