Is there any way to see which version of a dynamic email a prospect received? I can check their segment and make an assumption, but is there anything that can definitively tell me which one they received?
Good call - you can always group an Email Performance report by the corresponding segment. I'm not sure if Marketo has a native way to do this, but one potential solution is to create lists for each segment in your segmentation and add the leads to the corresponding lists at the time of the email send. That way, when you look back, you can know which segment each lead was in at the time of the send.
You can look at Segment + Was Sent Email. Probably will be accurate.
There is likely an Idea on this.
I was hoping there was a better method than this, if only because it's still not definitive (since segments can change), but it sounds like this is the only answer at the moment. I spoke with Marketo Support and they recommended I send myself a sample with the lead set as the prospect in question -- essentially the same thing as you've recommended.