how to tell a airbus a300 and boeing767 apart ?

Answer 1

I've found a very easy way but I don't know if it will work for you. Basically I'm an artist with an EXTREMELY keen sight especially when it comes to shapes. Almost all (maybe all) Boeing airliners have a distinctive shape of cockpit windows. The same goes for Airbus.
Have yourself a very gooooooood, lllloooooooonnnnnnng stare at (from outside) cockpit windows of 757, 767, and 777. They all share the same basic shape. Now do the same for the Airbus A300, A310, A320, A330, A340....notice any similarity on these sisters?
Now compare the Airbus windows with the Boeing windows.
Like I said you need very keen, critical sense of sight (with a good sense of geometry).

Answer 2

With so many aircraft having the two engine low wing formula it does get difficult.
The wing on the A300 is slimmer (higher aspect ratio) I have found that the only quick way to tell a boeing from an airbus is to check details like the pointed extensions behind the wing. These house flap tracks and other lumpy bits.
The airbus and the 767 are almost identical in size but the boeing has only 3 trailing edge 'lumps' on each side while the airbus has 5. Get an aircraft recognition guide and study the drawings and photos.
Pratice practice practice

Answer 3

The 767 is the one taking off with a load of passengers. The Airbus is the one in the hangar being fixed.

Answer 4

The data plates :)