I have a lamp socket that only has brass screws. How do I know which one is hot? I believe it is the the one that connects to the “tab” that the bulb touches.
Medium base E26 (1") Lampholders are normally rated for 250v and can be wired for US 240v bulbs with two hot conductors, so screw color for polarization is inappropriate. However, NEC 410.50 specifies that when a "grounded conductor" (which is the name the NEC normally uses for the neutral) is connected to a luminaire, it shall be connected to the screw shell.
So your thought was correct: with a 120v connection that leaves the center "tab" as the switched hot.