Is there any way to tell if an item is solid brass before purchasing it?

Answer 1

Please know that brass is a relatively soft metal, and will certainly show damage when scratched with glass.

"Here’s a tip. If you don’t have, you should get a small magnet to carry to test brass. A magnet won’t stick to brass, copper or pot metal. If an item is not marked and a magnet sticks, it’s plated not solid. BUT there are some exceptions/ Old brass beds can be solid brass but for added strength a steel liner in the posts and steel rods inside the cross pieces could have been added. This can cause the magnet to try and stick. Floor and table lamps will also have the steel rod usually to hold the individual pieces together and run the wire inside.
Brass color will vary too/ Most imported brass seems to be darker which comes from the amount of zinc used. The more zinc - the darker the brass.
If you need a piece of hardware for furniture, chances are you will never find that missing pull for your dresser. Save the old ones and get Good replacement hardware matching any existing holes if possible.
If you like your brass shining, coat with lacquer or wax right after polishing. If you like more “age” on brass and copper, wait till it darkens to where you like it, then coat to keep it from getting darker."

Answer 2

Take a piece of glass with you and try to scratch it with the brass. If the glass doesn't scratch then it is probably brass.