The XFi model metro was made from 91-94. So actually finding one does not guarantee it is still an XFi in anything other than name.
The main way to identify it is it has MS in the VIN instead of MR. All normal Metros and LSI models had MR as the 4th and 5th digit. There are some other ways to spot them but they are not always reliable. Both sections of the bumper are unpainted on the XFi model. Some cars depending on the state had no passenger mirror. A lot of the XFi metros say XFi on the hatch decal.
Once you have found a car that started out life with the right components you still need to check it out to make sure it is still stock. If the car is a one owner car they can tell you if the car is stock or what parts have been swapped over the years. If the owner has no idea then you are going to have to do some work to figure it out. If the car now has an automatic transmission then you know for sure it is not stock, there was no automatic transmission option for the XFi.
All XFi’s have a 5 speed with 3.79:1 differential ratio. So to test this the easiest thing to do is put the car in 4th gear with the timing mark of the engine on top dead center. Put a mark on the tire on the bottom so you know how far it has moved. Now turn the engine over by pulling on the car slowly watching the timing marks. Turn the engine 4 full revolutions. Now look where the tire is and it should have made slightly less than one revolution. 4th gear in the car has a 0.914 ratio so adding that to the 3.79 differential ratio means the engine should turn 4.1 times for each revolution of the wheel.
Now for the engine the only real way to tell is to pull the valve cover off and measure the cam. Nobody is going to let you do that to their car so really you are kind of out of luck with an easy to spot way to identify an XFi engine. The best you are going to be able to do is to check the engine carefully for signs it has been rebuilt or swapped out. Look for bolts that don’t look like all the rest of the fasteners in the engine. Stuff like missing bolts or wrong bolts in the bellhousing, exhaust mainfold, oil pan, and the other engine covers can give you a clue to what work has been done to the engine and if it looks like someone has swapped it out. Remember these cars are getting old so just because some of the covers have new gaskets doesn’t mean the engine has been swapped out. It could just be normal wear and tear.
Even if the engine has been swapped as long as it still has the XFi camshaft then really it doesn’t matter. The XFi engine had 2 ring pistons instead of the normal 3 rings. That and the camshaft are the only differences. The cam is the most important part though, the rings don’t improve the mileage as much as the cam, and you can always leave the second compression ring off a normal 3 cyl engine to copy the XFi pistons. If the engine has a normal metro camshaft and the XFi cam is gone there is really no way to replace it other than watch ebay and teamswift to see if one shows up for sale. They usually aren’t cheap though so try and find the right cam with the car if possible.