How to tell if a lens element is the facing the correct way?

[Airborn]

Hi everyone, I have enjoyed reading the forum but never posted before, please be gentle if I screw up!

I have a rather nice Chinon f=135mm 1:2.8 but the aperture was gummed up with oil, luckily taking off the back parts and rear lens element is pretty simple, so I used a swab and some Isopropyl alcohol to repeatedly daub on Isopropyl and open/close the blades. Seemed to sort it out, I put the lens back together used it, left it for a week and the oil was back, aperture gummed up again.

Anyway so now I'm in a rush and I know how to open the lens so no being painstakingly careful and documenting every step/the position of things, because of course I also know how everything goes back together now(or so I think) - except the damn rear lens element, flatish on one size - convex on the other, I have no clue which is the right way. I put the convex facing in and put it back together and the pics seem fine.

Is there an accurate way to tell which is the correct way the element should face?
If its facing the wrong way you should immediately see noticeable distortion in pics?

It's this lens here:

[calvin83]

[bghomofaber]

Putting it the wrong way would alter consistently the complex calculations that are behind the lens' project.

You would be unlikely to obtain a still working combination by putting the lens back in the wrong way.

If the convex side faces the viewer you are safe, since afaik only a few wide angle lenses have a negative concave front element, while there are no schemes with a flat lens surface in front of the lens (mirror tele lenses do not have a lens in front, just a flat optic glass)