How do I find a drain on garage floor?

If you have a copy of the original floor plan, it may show the location. i am not sure what kind of acrylic surface you are talking about, but could you possibly tap in the area where the drain was and see if you can hear a hollow sound?

the drain would have been put in a slightly shallow depression. If you run/dump a little water near the center of the garage, does it pool/run in a particular direction? That would indicate where the drain might be.

Check out any old pics you might have so you can get the closest estimate possible on exactly where it was. Then I would try tapping around, there should be a difference in sound where the drain is

If the acrylic surface was not a 'self leveling" one.. take a marble, ball bearing or golf ball and gently start it slowly rolling across the surface of your garage floor... it should circle and settle into the depression where the drain is.

I would use some sort of metal detector, like an electronic stud finder that has the extra light for finding metal. or you could try a strong magnet that might be able to find it through the epoxy. My client had a big circular one that we put on a string to drag across the ground to pick up screws from debri.

I doubt the depression thing would work cause epoxy is self-leveling so it gonna be thickest where you covered the drain. Don't do that again. Next time take off the drain cover and put a roll of wadded up paper in there above the floor level, then you can put the cover back on when the floor dries.

Use a level, the drain should be at the lowest point. Alternatively, a hand held heat sensor, like for use when barbecuing might show a slight difference in where the drain is, especially if it is metal. But I can hardly believe that they covered it, since that would have been more difficult than to acrylic around it. If yours is like my garage, maybe a box is covering it ! :)