I will set it out as follows:

The more diplomatic units sent against you have an either success or fail ratio. In this understanding we will deal with the 'failed' aspect. We know in a 'failed' attempt a city captures or kills the invading units. My suggestion is that the higher the amount of diplomatic units sent  the higher the probability to gleen enough information as to whom to sending party was.

 

This could be done as part of the counter-intelligence research that is already in game or another research option that can be added.

 

Again, my suggestion is that this additional aspect does not not guarantee knowledge each and every time only a higher probability to gain information in relation to the amount that was either captured and killed in the attempt.

 

Just some thoughts.

Belargyle