Joey, When the system is cold, switch heating on. Feel both pipes that feed the radiator. The one that starts to heat up first is the 'flow', the other one is the 'return'.
The system will have been drained down when the boiler was replaced , it is possible that a certain amount of sludge has found its way into the radiator, blocking up the bottom section of the radiator when the system was re filled, although at least two system flushes should have been carried out ( one cold, one hot ).
Suggest you ask your installer to call back to rectify the problem.
Hope this helps.