Looking a some pictures what's the difference Between the two ships?

It depends on what time frame you're looking at. As built they were very similar. The Graf Spee carried a top mast abaft the tower mast, (main superstructure) and two small shoring masts that ran from the main, (aft) mast forward to the stack. There were some minor differences between the ships as far as the main superstructure. The Graf Spee also had slightly thicker belt armor, but I doubt this could be seen externally. After 1940 the Scheer's superstructure was cut down considerably making it thinner and narrower to make it harder to see at long range.

Off the top of my head, (and my apologies if I'm not 100% accurate), because I've no references handy the most obvious:-
Speer had a modified bow, different searchlight and boat placement and (as the war progressed) more AA weapons and radar.
TTFN
Tom

Edit:
Andwhat Rodger said

There are 700th scale drawings of the Graf Spee,and the Scheer,in the Shipcraft book on Pocket Battleships.These include the various modifications made to the Scheer during the war.
Regards Phil

Something not mentioned yet - There was 2 rows of windows of the front of the tower - on one of the rows Scheer had 3 windows, whereas Graf Spee had 4 windows. Small difference but noticable.

thanks
Mike