The way i usually tell if a vehicle has been crash repaired is pretty much go over the whole car with a fine tooth comb (thoroughly). Check all seam seals (cracks, inconsistancy, different on one side to the other), check all edges for fuzzy paint edges, check all window seals for oversparay (especially on 1/4 panel - roof area, check this area for paint fade-outs too), check panels for inconsistant peel, dullness, defects, shrinkback ect. check for colour differences from panel to panel. make sure the car is in a well lit area and look on an angle down the sides, check for rippley/wavey panels. make sure all gaps are consistant. make sure doors, bonnet, boot all open/close without problems. Always good if you can check underneath the car too, chassis rails, under guards ect. It takes awhile to check all these things, but well worth it in the end, hope that helps.
sam