Deducing the CAD program used to make a schematic
The colors, style of library parts and such like are a fairly good tip-off, but usually they can be changed. So it depends on how much the designer stayed with the defaults or change them to comply with a company style or with personal whims.  

If you look at a reasonably large number of schematics that are created with various different known programs you will soon be able pick up on the differences and similarities. 

You can also try google image search for things like directives that you may not recognize, but I am not very optimistic about that working.