Having owned and repaired a couple of examples of the Auto S2, I would suggest that unless the camera has been tampered with it is reasonably likely the lens focus will be OK. It is easy enough to check with a ground glass and loupe over the film rails. If the lens focuses at infinity with the focus set to infinity, there should be no reason to expect that shorter distances as marked on the focus ring will not also be correct. If they were not the only reason for this to possibly be the case would be that the ring is not correctly calibrated--unlikely. Having said this the focus ring scale and DOF marks of the S2 are not the most substantial affair ever devised and seem more of an afterthought, but nevertheless.
Note that you've got two separate focusing systems in play in your camera. What the film sees--the primary focus through the lens--is the master focus. The rangefinding system must be calibrated so that it matches the distances focused by the lens. The same principle applies to other camera types (Eg. TLRs have two separate lenses that both have to agree; SLRs both share the same lens, but from the mirror onwards, the viewfinder system is semi-independent of the film focus; and so on).
Having pointed that out, in the case of rangefinder cameras in particular, whilst it is possible that the lens focus itself may not be correct, in the majority of instances, where there is a discrepancy in the focusing accuracy it is going to be because the rangefinder system has gone out of adjustment. Rangefinders have many good points, however one problem they are susceptible to is that their viewfinder focus systems are more prone than most camera types to being knocked out of adjustment by being knocked, dropped etc.
Of course, I can't completely rule out the possibility the infinity stop of the lens needs to be adjusted, however it's far more likely that the lens focus is still accurately set, and that the rangefinder merely needs to be brought into adjustment with it.
This isn't terribly hard to do with the S2. There are two adjustment screws; one set into the side of the viewfinder eyepiece, and one underneath the accessory shoe top plate, which may, with care, be prised up and slid out from underneath the shoe rails to provide access to the adjustment screw. You can even correct the adjustment yourself.
First, though (and I cannot stress this strongly enough) it is pointless trying to adjust the rangefinder patch, unless you have previously confirmed that the lens, when set to its infinity stop is actually correctly focused at infinity. If you don't verify this first, and the lens adjustment is off, you will just be chasing your own tail trying to make the camera focus well at all distances. The lens focus is the master adjustment, (and hence, must be performed first). The rangefinder can only give you usable results if the lens itself has first been properly set.
Assuming you have checked the lens focus with ground glass and loupe and it looks right (use a power pole, tree, or other distant object Eg. the moon, to critically check the lens with the shutter set to bulb and the aperture open fully) you can adjust the rangefinder until the patch aligns with the main image at an infinity target. Again any object more than about a half a kilometre away for the S2s 45mm lens will be good enough (longer distances will be required for longer focal lengths) or once again the moon is a good infinity target if it is bright enough (it's close enough to infinity for all practical purposes, obviously!).
Once you have the lens focus confirmed and the rangefinder patch dialled into that you are good to go. The other screw by the finder is for the vertical adjustment, S2s are often OK in this respect but if needed this may be fine tuned as well.
Cheers,
Brett
