727 valve body,

You can't go by the separator plate unless they drilled it, and that might tell you if there was a shift kit of any decent type. (Unless the plate was part of the shift kit)
The 2nd and sometimes 3rd gear fluid supply holes are usually drilled larger.
The other tell tale sign is a lot of the shift kit springs are brightly colored and factory springs not so much.

Tracy Fishy68 might be able to tell you a for sure way to tell, but I can't really think of anything else without comparing hole sizes, grind marks, or spring pressures.
Maybe if it has a non stock manual valve (the one that circulates fluid in Park) or the valve body fluid track wall is drilled out.