Fluke ST120+ GFCI Socket Tester with Audible Beeper

Q: Does this work to simply test if an outlet is working ( I'm not an electrician, so in other words if it's "hot" ) ? Even not talking about gcfi.

Yes, this outlet tester will verify that there is current flowing in the socket ("live"), but importantly, it will tell you if the socket is wired correctly. Sometimes electricians or "DIYs" wire receptacles incorrectly, putting the neutral wire to the hot terminal and the hot wire to the neutral terminal, which will work but which will make some electrical devices unsafe to use. This will tell you immediately if it is wired correctly, it primary function.

Q: Can I attach a 3 prong adapter to my 2 prong outlet then test with this device?

The whole point is to test all 3 conductors to assure they are in the right orientation. But, I bit and tried it out of curiosity. Drum roll please - Open Ground! But you probly knew that.