It will look like...clean glass. That's it. The film of old silicone you show in that pic, if not removed, will negate all the work you have done by causing either tank failure (if it extends into the seam) or possibly a leak (if it is just under the bead). You keep asking how can you tell if the glass is clean, how to tell if the silicone is gone, is this good enough, etc. Come on...you know that's not clean. Clean is...clean.
And, although I am aware that repairing this tank is an emergency of the highest magnitude...I would strongly suggest that you get some rubber or latex gloves before working with acetone. A 15-second google search reveals that acetone...and other solvents like mineral spirits, xylene, toluene, etc. "may" help to soften cured silicone. Interesting bit of info, didn't know that. But...please remember that these are dangerous substances, contact with which can cause a variety of chronic and acute medical problems. Using them safely demands proper gloves and a well-ventilated work area, or preferably a respirator.
Don't work with bare hands. This is not one of the stupid things you do because it's also exciting. It is just a stupid thing to do, period.
Glass can be gotten completely silicone-free with the use of a razor blade, and perhaps some emery cloth on the edges. It sounds like you have already done almost all the hard work already. If you must do this RIGHT NOW, you should be able to finish the job the same way. But, if you insist on going the solvent route, please do yourself a favour and do it safely. If that means waiting a day or two for a pair of heavy gloves for working with glass, or latex gloves for using solvent...then wait.
