Listen for the solenoid click when you start the dryer. No click usually means a bad thermal fuse. If you get a click look through the peep hole. No glow usually means a bad ignitor. If it glows but then goes out without lighting the gas, bad gas valve coils.
Other things can cause no heat too, like the timer, flame sensor, motor or limit thermostat. The thermal fuse, gas coils or ignitor are far more common.
There are a lot of videos for installing belts like this
. When you get enough practice you don't even have to look. Just stick your head in the drum and install it by feel.