There is a handy form under system administration called the element usage log. This keeps some statistics about how often the form is accessed. You could use this information to determine if it’s worth upgrading a customization if you are looking at moving to AX7 for example.
This doesn’t tell you which users might be using a form. If you are looking for that detail you could have a look at the client access log. see the post.
Here is a quick overview.
AX2012 R3 CU11