Sounds like you're referring to the in-tank seal that you can actually feel, correct? If so, then the only thing to check for, is to see if you can physically lift the silicone off the glass. Poor prepping of the glass will result in poor adhesion and will cause the silicone to come off the glass easily. You can have an ugly looking seal and still have a proper seal, and you can have seal that looks factory done and its actually a poor seal because the prep wasn't done correctly.