Check the dial knobs that adjust the monitor, make sure they all change the screen display as they should.
Check the wiring going to the joysticks and buttons, a joystick is 10$, buttons are 2$. If the wiring is good, you can get new controls for cheap. Make sure to test each slot.
Dont test the buttons by playing a game, if you hit the neo geo hardware options menu, there is a screen to test all the hardware functions, including every button and joystick direction. It can also show you red/blue/green blocks to help adjust the video.
Check both coin slots, and make sure the credits appear on the screen and the lcd above the joysticks. seems like a no brainer, but remembering the coins is the big part.
Locks are 4$ each at lowes or home depot, its common to have them removed before selling if they share a key with other machines in the route it came off of.
so dont worry if those are gone.