I ride a 2006 Giant Cypress SX with 700x40c tires. I'm planning to get Continental Gatorskins to improve road and puncture resistance (first flat last night).

One LBS told me that narrowest tire that will fit on my rims is 700x28, another LBS recommended a different size.

Is there a way to determine that myself? Is there a marking on the rim that tells me the range of tire size that will fit? (I'm not at home to look at it now.) I'd like to order on-line ASAP.