All Boeing 757's look the same when you watch them at a quick glance, but if you look at them more closely you will see that they are not always the same one from the other, the most distinctive feature is the power plant. The 757 is offered with 2 different engine types: Pratt & Whitney and Rolls Royce. The engine choice is up to each airline. If you see a nozzle coming out of the engine cowling, it's a Pratt & Whitney. If you don't see a nozzle coming out of the engine cowling, it's a Rolls Royce. American, British Airways, Continental and US Airways use them with Rolls Royce. Delta, Finnair, Northwest, United and TWA use them with Pratt & Whitney. Just to give you a few examples of who use them with which engine. Of course, the airline customer number (the last 2 digits) tells you right away who the aircraft is built for, but looking at the engine also helps you identify what airline it could be. A 757-223 with Pratt & Whitney engines, that does not exist, neither does a 757-232 with Rolls Royce engines. A 757-222 with Pratt & Whitney engines, that does exist.
DeltAirlines, a 757-236 is built for British Airways, not Britannia. A 757 built for Britannia would be a 757-204.
The other feature that could help you identify what airline the 757 belongs to is the seating layout although all 757's are arranged as follows: 2-2 in First and 3-3 in Coach like on all other narrowbody aircraft. But, the number of seats in First Class and Coach can vary from one airline to another, depending on how each customer wants to plan the aircraft. American has 22 seats in the First Class cabin on the 757.