Currently there are two ways to tell which fields are Primary Fields:
Option A: Go to Tools > Define Fields, then individually open the field and check to see if "Primary Field" is checked
Option B: Run a Field Detail report in Act! Diagnostics and swim through thousands of lines of text that, looking for the correct table and the correct field, and that line of text that says "Primary: True"
Both options are rather time consuming, and while Field Reports are useful for those who are a little more technically savvy, Option B is not very end-user friendly. I believe that this could be made more efficient and user friendly if a Primary Field column could be added in the Define Fields list, much like there's already a column that explains what Company Field a particular field is linked to, if any. For example:
The column could go next to the "Linked to Company Field" column and it would make it much faster to discern whether or not a field is a Primary field.