ESD safe brushes are usually made out of carbon fibre which is electrically conductive.
I have bought ESD brushes from Aliexpress before, the first lot I bought at least had conductive handles in the range 5-10M, the second lot a couple years later appear 100% identical but do not show any conductivity at all, so they went in the dubious junk drawer, not being able to readily test them to see if they are just dissipative.
Now there are higher count meters readily available for everybody it's easy to test quite high resistances, but if you only have a 2000 count meter or whatever a simple trick is to use an LED and a (preferably current limited) supply capable of 30v or more, wind the voltage all the way up and use the handle of the brush as a resistor in series with the LED, even though some number of meg ohms a dim glow can be seen in the dark.
Not that it necessarily is of any real concern in a hobby situation, but if you're buying an ESD brush from China then you might as well check to see if it actually exhibits any property that is consistent with being an ESD brush.