Hello gas_treater,
This is a good question, but with unfortunately only few answers.
You must have a history of hydrocarbon concentrations for the specific plant and you will be able to determine the 'typical' concentration increase of hydrocarbons which are contaminating your expensive MDEA. When the concentration is 'flat-lined', or there is no growth, there is a good chance the charcoal system is working properly.
When the system is not working properly, or a large 'slug' of hydrocarbons contaminated the system, unfortunately, the system should be emptied of the contaminated fluid, the system cleaned with a degreaser, and the system charged with fresh MDEA.
Activated carbon should not provide a delta pressure (if it does, you have MUCH more serious problems than hydrocarbon contamination).
Foaming frequency can be used, as long as the foaming is coming from hydrocarbon contamination ... this can be hard to determine sometimes.
The best solution is to prevent the hydrocarbon contamination in the first place (either by sizing up the knock-out, or treating the hydrocarbon problem in the field prior to reaching the plant).
I hope this helps, and I wish you the best.
Cheers!
Eugene