I'm tuning up a 2010 Giant TCX 2 which has Giant CR41, 32-hole rims as the only information I can find. There's nothing printed on the rims themselves either.
Right now it's using the 700 x 35c Kenda Small Block Eight folding 120TPI tyres that it came with but I'd like to change to slicks and have found some 28c's that are an amazing deal.
As per Sheldon Brown I'm likely ok putting on 28c's since they fall within the range of rim where you'd find 35c's but not always.
My ability to measure this at home is limited by my lack of a ruler. Surely this information is available somewhere.