2 pipe steam

Alan_6

I dont know of any other way, besides taking the trap apart, and taking a look. It could be clogged at the base of radiator, the trap, or the return line.

Chris V

Thermostatic traps

On 2 Pipe steam radiators, how do you tell if the thermostatic trap is stuck closed? Radiator won't heat.

Fred Harwood

Traps

Radiator trap bellows often expand on failure, and close the outlet hole, which eventually nearly fills the rad with condensate, depending upon the condition of the outlet seat. If a large rad, you might also have had to add water to the boiler after the trap failed.
If the rad has water in it when you open the trap, replace the internals. Then check your boiler water level.