I'm not sure why you want to do this, but you can check the degree of utility balance two ways.
the first way is sort of indirect, but I think it tells you what you want to know:
If you've built a simulator, you could just simulate shares for the holdout question. For example, let's say your holdout had 3 alternatives. To the extent the choice set was utility balanced, your shares should be 1/3, 1/3, 1/3 . To the extent your shares deviate from an equal 33.333% for each alternative, your utilities were imbalanced.
I suppose you could also sum the utilities for each alternative in the holdout choice set and see how even they are, but because different respondents answer with different amounts of response error, I think this direct measure will be a lot less intuitive than the share-based measure above.