How to assess participant awareness of experimental deception without inducing awareness?
Procedures like this are often called "funnel debriefing" procedures, and they basically consist of an extended version of what you already had in mind.

You begin with some very general questions about the nature of the experiment (e.g., "Do you have any initial thoughts or reactions about this experiment?"), then move on to some questions that are slightly more specific, but still not fully revealing (e.g., "Why do you think we showed you X?", "Did you notice anything unusual or inconsistent about task Y in the experiment?"), and finally terminate with direct inquiries of awareness (e.g., "Did you notice that we swapped one of the images during task Y?").

These questions may or may not be accompanied by recall or recognition tasks where awareness is gauged through participants' performance on the memory task.

Unfortunately there is typically little or no standardization (at least that I have seen) of how extensive these procedures should be, or for how to score and otherwise handle the results.