There does not seem to be a way to determine whether one has used media in the collage or not. Maybe it comes to the small number of clips/images you can use in a collage, or maybe to importing sets of clips per time in a well-organized manner. It is a serious omission, and adding a used in collage indicator, like we have a used in the timeline indicator, would improve the user friendliness of PD.
I suggest you put forward a suggestion to CL (they claim not to read anything on the forum) using the
rate us & provide suggestions.