It should say on the front if it's dual-purpose. Is there a way to change the torch polarity? Usually there's big terminals which you simply swap the cables over, they'll be marked + and -.
I think I'm right in saying that almost any MIG welder (transformer-based ones, at least) can be converted to gasless. It's either:
1. Because convenient features have been built in by the manufacturer to make it easy to change polarity, or
2. If you are DIY-minded and realise how easy it is to carry out the very simple wiring changes yourself, even if the machine is described officially as 'gas only.'