If you look just above rear brake pedal that intersection  on the frame is the first clue its probably not aftermarket. Its identical to OEM, most of the aftermarket frames were built 92 or Honda geometry and have a removable sub frame.
There were a couple early pre 90 frames but I havnt see one in 20 years. Graydon Proline made one and C@J may have been the other, they were one offs.