open the prt file with a pure ascii text editor (notepad++ is great).
look for this string:
pro/engineer tm wildfire
(note that is next to a zone bound by characters #)
you will see the version of pro/e with which the last version of the file has been saved at that string
(in the case mentioned #pro/engineer tm wildfire 2.0 (c) 2004 by parametric technology corporation all rights reserved. m280).
