Are the 2.5 and 2.7 litre 944NA engines different externally?would you be able to tell one from the other just by looking at it?

 

Not really anything visual except the engine # stamped on the case:


Also, when compared to early 2.5's, the oil pan has the lower sump housing (round flat bit when looking underneath) This started in MY88 though, so not a tell-tale... also, many have been backfit according to the Porsche TSB.

For more info on engine ID's, check here:
944 engine identification