How can I verify a supplier is a real factory and not a trading company?

Ask for a live video walk-through rather than a recorded tour. Look for three things: a cutting room with fabric rolls, cutting tables and markers; work in progress at every station on the line; and the actual number of operators present. Ask the camera to show the factory gate and the business licence on the wall.

A trading company can show you a showroom, a catalogue and a sample room. It cannot show you a cutting room with your fabric laid up on the table.