Externally visible differences between Bf 109E-3 and E-7?

[lritger]

Andreas pretty much nailed it... if the cowling is off, you could look for the "saddle tanks" on either side of the engine, which appear to be a hallmark of the DB 601N and that would be one way to identify an E-7, as would an external fuel tank. But as Andreas points out, so many early models were rebuilt into later variants that it's often an exercise in futility to positively identify what a given aircraft is, even with the W.Nr! One can make an educated guess, but that's about it.

Lynn