If it really is DC:
Tools with universal motors will be OK, probably. If it's got brushes showing, and it's designed to plug into an AC outlet, it's a universal motor (probably).
Electronics with transformers will FRY. I would expect a popped fuse (or irreplaceable fuse-like thing in consumer electronics), so probably no interesting fires to write about.
Dunno what would happen if you plug a nice expensive super-gamer PC in there -- don't come crying to me if you do and break it.
If you have a voltmeter measure the DC voltage and the AC voltage. If it's really a DC thing then the voltmeter will read DC (and possibly not AC, although no promises). An AC outlet would read little or no DC.
If you can see the neon bulb in your AC tester, look to see if both of the the little rods inside are lit, or only one. Only one of them ever lights at one time (I think it's the + side, but I won't promise), so if it's DC only one will light ever, while if it's AC they'll switch back and forth at 60Hz and you'll think they're both lit.