Actually, there is a good way to test that. Fill a clear bottle half full of soil, then fill it most of the rest of the way up with water. Shake it up a whole bunch, then let it sit for a few hours. There should be some really obvious delineations between sand, silt, and clay. Clay will be the top layer, and if that makes up more than a quarter to a third of the soil, then there's too much clay.

Small gardens are popular in Asia, at least in Japan. I can't imagine they buy crappy soil.