How can i tell supply from return?

I am replacing a small radiator in my kitchen with a kickspace heater.

I have a 4 zone boiler with individual pex runs to each raidator, what i cant figure out is if there is a way to tell which pex line is the supply/return.

Is the bleeder on the radiator always on one side? is there a simple way to figure this out?

 

If you can't trace the lines out turn the heat on from a cold start. Holding both lines the one that gets hot first is the feed. Can't see what difference it would make on a toe kick heater.

John