Yes, it's the Alcos that are hardest to state the difference between and the GE units! Maybe not surprisingly, given their technological similarities (turbocharged engines of similar dimensions, etc). Maybe there was even a similar design philosophy, running back to the days of the Alco-GE consortium: both, for example, put radiators at the end of the hood with the fans-- fans mechanically powered from a crankshaft extension on Alcos and pre-Dash-8 GEs, instead of fans with separate electric motors-- under rather than over the grids.
I'd point to the hood ends as well as the cab roof. GE (except for things like the UD18B and the first two U25B demos ("XP24") have rounded edges. 251-engined Alcos, even when they don't have notches, have at least one pair of sides (typically the end and the top) meeting at a sharp angle. (Of course this makes GE's soud like RS2/RS3 era Alcos! If your guide is to be used outside the U.S., you'll need a warning footnote about the Queensland (Australia) units from the early 1950s: they were a GE export, with an ancestor of GE's FDL engine, but have a carbody design startlingly reminiscent of an RS-3!)
One thing that helps, bu that you need practice recognizing: handrails on GEs are made from a larger diameter pipe than those on EMDs and Alcos. But you're right, maybe the best thing to do if you have just one drawing is to show the cross section of the GE (U25 thrugh Dash-7) cab roof.