If you're attached, ask them what fire precautions they propose to the party wall and soffits. This gets them every, single, time. lol. And a "none" answer gets nil points.
"Do you contact building control or me" - just that one sentence randomly thrown in gets a good indication of if they know their stuff or not - and the answer is not just a "you" or "I do". You are looking for mentions of insulation, rafter loading, fire safety.
"Got and RAMS?" often gets blank faces. "Ewe must be joking" is not the answer either.
"What are you proposing for the eaves?" You want to hear about eaves support tray, comb fillers, rafter roll and if they tape the membrane.
"Do you make sure the membrane is nice and tight? The answer is "No we don't", and an explanation why.
"Are you fitting air-open (AO) membrane?" Ideally, they should be.
"Are you asbestos aware? - training. Along with "do you do the asbestos survey or me?" "What do you do if you find asbestos, and where might it be, how do you dispose of it?"
"What insurance cover do you have and can I have a copy of the certificate?"
"Do you use a manufacturers system to get me a system warranty?"
"Do you have an insurance-backed warranty?"