There are a number of small visual clues, the steering wheel is more modern on the TR250, as are the seats. The grille has some matt black paint added on the TR250, the side chrome strips from the front lights to the doors are wider and the trim strips at the crown of the wings/fenders are painted body colour on the TR250, left polished stainless steel on the TR4a.Some early TR250 have a distinctive and ugly "mag wheel" trim over the disc wheels and most if not all have a brake servo. I think they are very similar mechanically, other than the engine.
Simon.