Hey Troy, good point. The way you tell is read the manufactures spec on what rim they say the tire fits. They all have those specs, just sometimes they are not easy for the regular joe to get, the dealers all have access. Some of the same size tires, stamped on the sidewall, will fit slightly different ranges of rim widths.
This is true for all tires for all vehicles. The same size from one manufacturer, even within models of the same manufacturer can actually measure very different. That being said, you can get the info on the measurements and decide if it is close enough or not.
PJ's point was valid for safety reasons, if you cram fat tires on skinny rims or vice versa you are changing the dynamics of the design. It may not change it enough to kill yourself, but then again, it may... He is just blunt about it ;D
Like anything in life, take it with a grain of salt and make your own decision, just educate yourself first. I have a wider rear tire than factory on my 450, becuase the recomended honda size is no longer made by anybody but Dunlop and I didn't like that tire, but the one I chose was designed to fit my rim width, so I can do the ton and shouldn't have a tire related issue. But some people would say I shouldn't be runnig that tire becuse it is different from factory spec. Just like putting a directional tire on against the arrow, Some tires just won't handle as well and that's it, but some can completely come apart because the stress is applied opposite to the design. Mostly bad if you do "sustained" high speed runs like track or freeway. 50 MPH on back roads and you would never have an issue. Just be aware and be safe. My roadriders have an arrow and the guys installed it wrong the first time, I checked with the local avon rep and he said make the turn it around because the tire was designed to shed water outward when running the proper direction, if I left it, the tire would never fail but every time it rained the water would go inwards instead and be forced into my back tire causing really bad things. I went back to the shop, said hey this tire was put on backwards and they fixed it no charge, so that was good, but I could have been in serious trouble the first time I got caught in the rain. Not even a week after I was on a ride from my house to my in-laws, 1.5 hour trip and hit a thunder storm that dumped 40mm of rain on the roads in 1 hour, the tires were great, I could ride a whole 20 MPH with water up to my footpegs and stay on the road ;D, when the golf ball size hail started falling I found the first place to get off the road. ;D
And remeber I am not a tire expert!
Cheers,
Maritime