Last time we went to the US (the first time together since we were married) we couldn't find anyone to ask....so we got in the US line. The guy at the end of the line then directed us to a non-US line together.
When we got there the IO explained it was up to the airlines (that's who the guy on the line works for) which line you went into but they did prefer married couples to go through together so they are aware you were traveling together.
From a Britons perspective it was the quickest and least stressful entrance to the US I've ever had. Very little questioning, it certainly didn't make me feel as though I was being interogated as it has in the past.
Coming into the UK, again they try to put you through the smallest line, which was great when we came back from Turkey on a holiday flight and they allowed me to go with DW in the non-EU line and we were straight through!
So basically, if there is an official stood near the line just explain you are a US citizen married to a UK citizen and ask them