Well, if you take a particular I-frames of the encode and compare them with the corresponding source frames – you won’t learn a thing. That is because, I-frames are treated with great sanctity; and thus the encoder tries replicates these frames as faithfully w.r.t. the source as possible even when you use a relatively bad setting.
A B-frame, on the other hand, takes advantage of compression techniques the most – so B-frames are the best indicators.
When a frame that was P-frame in source [P-frames are space-heavier than B-frames]; but is encoded as a B-frame in encode – such a frame best indicates how much work [or damage] your encoder has done.
When a frame that was P in source and B in encode resemble each other – that means that your encoder has created an encode that is very faithful to the source. You have achieved transparency.