Trill the assumption is just made that the titres are high enough to provide immunity. I don't think docs would test for immunity simply due to the cost!
I would like to know the rate of successful immunisation though. As every single child I know that has had Chicken Pox has been imunised! In my daughters class at school out of 21 children only 5 are immunised. 3 of those 5 had CP a few months ago - none of the unimunised ones did!

So maybe the CP vax isn't very successful at sero conversion?