how can i figure out the resonant frequency of my room?

Your best bet would be to get the sine wave generator (and some ear protection), hook it up to a decent amp and sub/ bass speaker and sweep the frequency untill you hear a peak, probably in the high teens or low hundreds of hertz.

Knock yourself out... I dare you :D
resonant frequencies

I'm not sure how far you'll get, other than possible hearing damage. I know those car audio guys find the resonant frequency of their automobile interiors and go to 150-160db+ and still have their windows when its over. Maybe they actually modify/change their windows so they don't break? Which is another thing, you may need the resonant frequency of the glass door and not the room?
Actually, you're most likely not going to need to get loud enough to experience hearing damage...

As posted above, get a tone generator and turn up the volume to moderately loud and slowly sweep the freq upward. You will hear the resonant freqs in the room because things will buzz and at certain freqs the system will seem much louder than at others... That having been said, keep in mind that the response of your ears is most likely not flat but you will be close...

Another option is to use a spectrum analyzer and white noise. This is how we used to tune rooms when I was doing pro sound work. The analyzer will clearly show you the peaks that the room is putting off and has the advantage of also showing you driver interaction as well as whole system interaction.

Lastly, be careful if using a tone generator with the volume too loud. Keep in mind that it's creating a continuous wave and as a result doesn't give the voicecoils in the drivers time to cool. If it's loud and you're sweeping low freqs the bass driver voicecoils will get very hot!!!! BE CAREFUL!!!! I needed a recone on an 18" driver once because I was testing at low freqs with a very large amp and got carried away... Please don't repeat my mistake!!!

This should get you close...

Good luck,

The easiest way is to calculate the wavelength of the room--the length, that is--then place a speaker at one end. You can also go for half, quarter, etc. wavelengths.
Ezmeralda's point about the glass is a good one, though. The resonant frequency of the glass will be quite different fromo that of the room, and better damped, as well.
Not that I'm recommending that you collapse your eardrums in this manner.
There used to be a bass rig--the Acoustic 371--which had a small red and white warning label on the back of the cabinet saying that SPL within 10 feet of the cabinet could exceed 130dB. Naturally, I had to try it...
Looking back, I wish I hadn't...

can i explode my skull if i find it's resonant frequency?
It possibly can once you realize there are many resonance frequencies.
For a rectangle room without furniture calculations should be pretty exact but of course reality is different. However it is a nice tool to correlate measurements and theory when you do speaker measurements inside.

Just plug your numbers into the sheet (modes1.xls) and look astounded at all those resonances in your room:
Room Acoustics
Search for "modes1"; its about half way down although it might be best to read it all.