I remember a time when you could get the bill scanners for about $20.
DIY Quarter accepters were about the same. They used weight, conductivity, and magnets. If someone tries to use a "slug" (Fake quarter, or piece of metal the same size as a quarter, like a washer) in the coin slot, it would reject it if the tests didnt pass.
The coin drops onto a plate. The plate is connected with a shaft to a load cell a uC gets the weight and checks it for the proper weight tolerance.
There are electrodes (solder pads) on the plate that test for resistance/conductance.
A magnet is energized, if the coin lifts, you know its fake. You check for lift with the same weight scale. If the "quarter" gets lighter when the magnet is on, its fake.
A servo or solenoid can direct the coin, when dropped after testing, into a slide to the coin return, or the cash box.
Neat. And pretty easy with those stick on load sensors. The magnet was a nail with some copper wire wrapped around it.
Easy and cheap to do.
If you have a CNC machine, or access to one, you can make a professional or artistically done "face plate" for the coin op.
The same friend who made the above ones, had a few cut from plexiglass. REAL neat. With some etching and some LEDs, it was quite "inviting".
You WANTED to put your money into it. Kind of like an exotic dancers g-string.