this has bean explained many times here before.
but the important thing to look at is the shape of the middle bar. on true STs it will (in 99,9% of all cases) sweep backwards at the bottom of its belly, i borrowed niteoners pic here (hope that's ok?) to demonstrate:
and on ITs the middle bar usually goes straight down and all the way under the belly like this:
and the thickness of the bar on ITs is also in most cases equal at the top and at the bottom.
...but these are only one way of many to tell the true ID.