Like someone said earlier the examination of the engines would be done using forensics.
The people doing the examination have certainly seen all kinds of bird ingestion damage before vs damage from other causes. In addition there are well documented historical evidence of high RPM and low RPM water ingestion assuming of course that the water got into the engines and wasn't shielded by the inlet lip
There also are techniques for examining for bird debris (if present) even after crash landings. So with all the possible evidences and techniques available lets at least wait until we can see an engine before speculating on what can't be done.