Originally Posted by FastJake

I was going to suggest that you measure them, but the difference between 700c rims and 27 inchers is so small you'd need to be really accurate. Here's what I'd do: take the front wheel off. Remove the old tire and tube. Take in into a bike shop and have them fit a tire on (without a tube) just to see which size it is. With a tire of known size it'll be pretty obvious.

Before I did this, I'd search each tire and look for markings. I know you said you couldn't find any, but they might still be on there somewhere.