Solid carbide is not steel. It is a tungsten carbide grit material that is held together with 6-9% Cobalt. Some alloying agents may be used too. The only reason it is slightly magnetic is because of the cobalt.

Tungsten steel has some 0.5% to around 5% tungsten as an alloying agent. It does not significantly alter the magnetic properties.

A sure fire way to tell if you have a steel or a tungsten carbide is, as already mentioned, the weight.

A 6mm endmill that is 40mm long should weigh around 9grams. A solid carbide will weigh around 17 grams. If you do not have scales to hand but you have time, dip the blunt end in vinegar and see which one rusts.