I don't think I've changed a part that I didn't check out.
I go to check in the assembly containing the part, and I notice that inventor's vault browser icon circle is unchecked and green.
I check in the assembly and receive the message:
I have no way of knowing if I should replace it or not.
There should be a show changes button on this page. When you click it, a window should display both versions of the file, with everything that is the same translucent, and only the differences in full detail, or highlighted in some other fashion.