Look on the rim lable. Most manufacturers list the inside width of the rim along with the actual diameter in mm. Check with the manufacturer or your LBS but in most cases you can install a tire that is as little as 3-4mm wider than the rim. My touring bike has 22.5mm wide Salsa Delgado rims (inside dimension) and I ran 25mm tires on it all last year (about 3,000 miles) with no problems. I switched to 28mm tires for the comfort this year, but wouldn't have any problem putting the 25mm tires back on if I had to.
"Those who say it can't be done should stand aside for those who are doing it."