in one easy step (as opposed to the 30 mins i was detained for while they confirmed the semi slicks were indeed road legal)
any road legal tyre in use in the UK has “DOT” (Dept of Transport) moulded into the side wall (inner or outer… careful… took copper a while to find the print on the inner wall)
The “DOT” you see on the tyre is actually to do with the U.S. Department of Transportation. The UK no longer has one of these! (Called something else now!). V. surprised a copper told you this. Kind of worrying! http://www.safercars.gov/Tires/pages/TireLabelPassVeh.htm
To be legal on EU roads, tyres must be type approved and “e” marked, like other vehicle components. So look for a small “e” followed by a digit which indicates the country it was type approved in. e.g. e11 for UK.