http://www.grahamgoode.com/cosworth/power.htm

The main giveaway is the rocker cover....the link above shows both...top one is Big T (BT) and bottom one is ST.

The ST is known as the YBP and you'll find this stamped arround the thermostat housing area on the front of the engine....

There are many other ways to be sure...some of which i'm not 100% certain on so dont want to mis-inform you on them...someone else will provide info i'm sure.

Cheers
dan