hello
Try to place two manometers, one on the inlet flange, the other on the outlet flange, air side.
Verify with the heat exchanger data sheet the pressure drop of the component, as indicated by the supplier.
then, if the pressure drop are higher than declared, your heat ex. has to be clean.
If You nead a continuos registration, for istance to give indications to the plant operators, you can use a pressure differential trasmitter, i.e. two pressure transmitter devices connected together.
In any case, if your H.E. is dirty, on the secondary side you should have less temperature (at the secondary side outlet) than usual, or guaranted (obviously if your HE works at the design conditions, with reference to primary and secondary flow and inlet primary and secondary temperature)
I hope to be helpfull....
Bye
Engy74