In FS:
Terran fighters ALWAYS look at least vaguely like fighters (yick)
New Vasudans are Symmetrical, smooth, and have that annoying scaly brown texture that's supposed to make them look cobbled from sandstone or summat.
Old Vasudans look cool as all hell, but generally follow the same guidelines as Terrans.
Terran capships look superficially realistic as ships, gut tend to have engines all along the bottom or something like that. Also, they're big and chunky.
Shivans look like everything that doesn't look Terran. In FS1, that means they look like a less-standard variety of Terran vessel, in FS2 they look like bugs and spiky things.