or try looking up your bike on bikepedia.com, though of course proper measurement is a better guarantee of correct information. If your bike is an old steel bike then chances are it's 27.2mm (it was a very common size), if it's a more modern alu frame, then it's very likely it will be a larger diameter (e.g. 30.9mm but not necessarily). Generally speaking steel frames have smaller diameter tubes than alu and hence seatpost diameters are usually smaller on steel framed bikes than alu ones. Alas, standardization of seatpost diameters isn't... standard.
I'm going to respectfully disagree with the poster and say that it's probably not a good idea to go to the nearest 0.2mm (though I'll freely admit I haven't tried it), particularly if you are oversizing the seatpost. Certainly with steel frames you can have an issue of alloy seatposts bonding to steel frames and being 0.2mm oversize won't help this. And I'm not sure I'd want to force in a seatpost in an alu frame either. I would think it's best to get the seatpost size intended for the frame.