Agreed, research is key. Small details like knowing whether a certain model has clear over decals and overall placement of decals is key. For repaints, tubing decals are often purchased from separate from the manufacturer decal sets, and if the tubing decal is not correct that can be another clue.

Some other possible indicators:

Repaints often will have thicker than original paint. This isn't always the case, but an unusually thick paint coat might raise a red flag. Obviously powder coats are thicker than paint, too.

Serial numbers. Sometimes the stripping and sanding of the frame in prepping for a repaint can dull the shore lines of a serial number.

Often restored bikes are not given sufficient time to cure, so things like clamp-on front derailleurs, brakes, and other parts which contact the frame can cause inordinate paint damage if installed prematurely.

Consistency of condition inside the frame. Some sellers may not be willing to open up a BB, but it shouldn't be an issue to pull a seat post and shine a light down the seat tube. If the paint is nearly pristine but there's a fair amount of rust inside the frame, that would be another potential cause for concern.

You will occasionally see paint overspray on a fork. If there's overspray on the steerer tube which does not match the frame color, that's another indicator the bike does not have original paint.