In some ways you are better off with someone who admits to inexperience but is clean and tidy around the boat and willing to learn/help.

I once took out a guy who was alledgedly a YM with more experience than Neptune .
Put him on the helm for 5 mins to put kettle on powder nose etc, from a broad reach he managed two violent uncontrolled gybes within 5 minutes.Proved useless going to windward as well.(I found out later he had in fact failed his YM practical and blamed an unreasonable examiner.....)

I think the point I am making in my rambling way is its impossible to know if they are any good [qualified or not] just by talking to people.A short sail will soon sort out the men from the boys.

As to bits of paper denying responsibility,they are worthless.If you are doing things right such as doing a safety briefing carrying correct equipment etc If someone still manages to injure themselves your insurance will take care of it if they try to claim.

If you are negligent or dont have suitable safety kit then an indemnity form will not help you if you end up in court.