Lol...
My system when kegging... I keg under CO2 pressure and push the beer into the keg through the "Out" post on the corny using a QD. So I can stop the flow very easily just by popping the QD off the post.
I have a similar issue with star san so I can't see the liquid level, and I've got foam coming out the top of the corny.
I dip my hands into the bucket of star san, getting them good and coated, rubbing them together, etc. Essentially my hands are as close to sanitized as I think is humanly possible.
I'll reach into the corny as I get towards the end of the fill, and find the "In" post with my finger. Once I feel the cold beer on my finger (i.e. it's a single finger-width from the bottom of the "In"), I stop the flow.
Works for me.
I do the same thing (push to the keg through the out post). I do it with the lid sealed, though, to avoid as much oxidation as possible. If you cold crash the beer first, you can actually see the beer level on the outside of the keg as you fill it by the condensation that will appear.