What game is it?

The coin door and the line cord are the most obvious signs. Depending on the type of game, sometimes the service outlet is European as well.

For the code, sometimes you have to burn new domestic ROMs, sometimes you have to set the dip switches on the MPU for the country.

There is some debate about this. For some, imports are less desirable. For others, condition is king--no matter if it's a domestic or import.

I really don't think most people care about whether a machine is a re-import or not as long as it works well. The types that do care if it's a re-import will sniff it out from the serial number, certification stickers, or whatever.

Also, if you're trying to disguise the fact that it's a re-import, you shouldn't. You should be honest about what you have and what the condition is. Anything else is going to end up with someone down the road calling you up two weeks after the sale and a bad taste in everyone's mouth.

WPC (TAF, etc.) machines have jumper wires on the board that can be removed. If the jumpers are open it defaults to USA setting. Re-imports are no less desirable to me. The game can be a mess or excellent condition either way!

I have had a couple. Often the power supply can be a bit hacked, I have seen some very marginal work with wire nuts.

Normally no service outlet for your tools.

I would still buy one without a second thought.

If it's set up for 220-240V EU/UK supply, then you'll need to change the transformer in the machine, or use step-up transformer to plug it into (probably cheaper).