If a 777 and a 767 flew over at the same time, could you idnetify which one is which? How?
777 is larger. Engine nacelles are larger. If the wheels are down, the 777 has 6 wheels on each main gear (i.e. the gear under the wings) while the767 has only 4.
Yes. I’m smart 
In addition to the above, have a look at the tail of the plane. The 767 has a rounded cone-type tail, whereas the 777 has theirs flattened, like they took the sides of the tail and smashed/pressed them together, similar to that of an MD-80.
BL.
It would be kinda hard if it was a 764 and a 772. But the best would be the size of the engines, and if the gear was down how many tires were on each gear.
I couldn’t. beyond a 1/8 mile or so they all look alike to me.
777 has a tailhook for carrier landings, 767 does not. Other than that, I’m with 185, I cant tell the difference over distance either. 
You are kidding, right.
uhmmm…yes I made a funny. Because if they are flying overhead…I would not be able to tell the difference. Sorry if I made you look to see if there actually was one. Oopsss …theres another one…a funny. 
By tracking it on flight aware!
There ya go!
Just to stir the pot…which I like to do 
and add my .02, If as suggested you follow FA tracking a particular airport you would know which airline flys the T7, and who is bringing the 76. Now IF the same airline, say UA flys both flavors… the belly of their T7 has what to me looks like an unpainted area all the way up its belly. Plus you can just tell the T7 is a bit wider then the 76, her engines are a little larger, the gear as mentioned…I would say if you live near say LAX, JFK, IAD, SFO…an airport serviced by long haul service just study the flights…GL!! 
nah! The -400’s have raked winglets. The wings shapes are slightly different. You’d have to look at the wing plan forms side-by-side for that.
But since when does a 772 and a 764 fly side by side… I get your point, just had to make a stupid comment. That’s what I do best. 
You don’t pick up on sarcasm very well, do you Will?
Hehehe
Like I said, have a look at the tail. B767s, like B757s, have the rounded cone tail behind the elevator. The B777 has theirs flattened together, like MD83s/MD90s/B717s.
BL.
Kinda hard to tell from the ground though.
Not at all. You can tell a cone shape from a flat shape from any point in the air. Case in Point:
The fuselage behind the tail elevator is very discernable between the two. One is flattened like an MD80, the other is round, cone-shaped like a B757.
By comparison with the B777, here’s note the fuselage behind the tail elevator on the MD80. Quite similar, wouldn’t you say?
BL.
Get your point, good examples though! By the way, love the Air Canada 773! 