Look at the rock. If it has been engraved, then you'll know that it is a cameo. It doesn't really matter what type of stone is used to create a cameo, so a cameo could be made using soapstone.
Feel the stone with your hands. If the stone feels like soap or like talc, then it is likely soapstone. Talc is the stone that talcum powder comes from, so a talc-like stone would be a stone that feels and looks a little powdery.
Weigh the rock with your hand and take a look at the color of the rock. If the rock is compact, heavy and gray but looks a little like talk, then it is likely soapstone. Note that talc and soapstone are very similar but soapstone is generally a grayish color while talc is generally white.