How can i tell if my tires are rubbing?

I have a 2003 kia optima. i put 18 inch rims on it and tire size 225/40/18. Discount said dont do it but 2 other tire shops said it was a ok size and did a test fit. (I was going to buy a set from them regardless) but just what discount said worries me. I have been driving and dont hear anything. Any signs or sounds I should look for?|||If your tires were rubbing you would see it,hear it and maybe even smell it. Hear it because it would make a noise by rubbing against something, You would see it because it would leave marks on the tire and whatever it was rubbing against, also if it was rubbing enough it might even cause a burning rubber smell.|||If one of them gets pregnant then your tires are definitely rubbing.