Bicycle Mechanics - How to determine what size wheels my bike is intended to have?

Hello everybody, new member here; first post. I recently traded a couple old off-brand bikes that didn't fit me, for a Schwinn Passage that is in pretty good shape. I looked on bikeapedia and learned a great deal about the components and such, and have determined I must have a 1994 model. The bike has a pair of old chrome plated steel wheels on it and I'd like to replace them with something better. However, bikeapedia tells me this bike was supposed to have 700c wheels, and the current 27" wheels have the brake pads pushed nearly to the top of the calipers (exage calipers and levers). Is there a way I can measure something on the frame and/or fork to be sure I'm supposed to have 700c wheels before I buy them? I will eventually upgrade the brakes as well, so even if the 700c's are too big for my brake calipers, I can work with that.

If the brake pads are crowding the top of their adjustment slots, the bike will certainly accept 700c wheels since 700c rims are 4mm SMALLER in radius than 27" rims. You will have to adjust the pads DOWN 4 mm and there should be plenty of travel to do that.

By all means get 700c wheels as the rim and tire selection is far better than 27" these days. Also, "alloy" (i.e. aluminum) rims will provide a tremendous improvement in braking performance.

Yeah, I know how much better aluminum rims work; I hadn't ridden on steel rims since I was a kid. I was not aware that 700's are smaller than 27's, I was made to understand it was the other way around; that's really good to know. Thanks for the info.

..I was not aware that 700's are smaller than 27's, ...

In the wonderful world of bike measurements a 700 AKA 28" has a 622 mm bead seat diameter while a 27" has a 630 mm bead seat diameter. Then there's a 635 mm standard that I don't remember right now what it translates to in inches and fractions.

There's a chapter about it on Sheldn Brown's site.