I, personally, know those (hopefully) few calls were I might have been blocked or something funky happened and in that case if a coach comes out, I have no problem asking my partner for help. At the same time, though, if the ump saw it clearly, they probably won't ask and you have to accept that.
As for a process... ask for time, when it is granted come out and politely ask umpire what they saw. If you disagree (which I'm sure you do, if you're out there
), politely ask if they would check with their partner. Again, it's the calling umpire's decision if s/he wants to ask for help or change their call. I have to say, the main thing coaches do wrong is come out of the dugout with a lot of hostility. Umpires tend to take coaches more seriously when they are not ranting and raving about everything.