I checked out my rims before I went to "E" rated because I would have more "reserve capacity" and our tire guy said that "E" rated are becoming more and more common therefore easier to replace if you ever need one on the road. I can't tell difference of ride, seems about the same to me. Steel valve stems are a big plus and only way to go IMO.
I didn't have any info on my rims either but a number and a logo gave me a start. I was able to find manufacture by the logo and the number was manufactures model number. Looked up info on manufacture's web site for model number and found out I was OK for larger rated tire. Some rims are stamped on back side, some on inside that you can only tell when tire is removed and some I understand don't have anything. Last time I was at a tire place I was looking at trailer rims (when having car tires done). And asked about how they tell if rim was rated enough. They couldn't give a good answer, but one answer was "they sell rims with a label that gives spec's" I noticed that they did in fact have a label. And I'm sure that sticky label would stay on for at least the first 50 miles.
I'm sure others will have different thoughts...Tire postings usually always do.