No real great suggestion, because even the customer quiet often doesn't know there latest revision and I am not trying to be funny here!. This is just reality.
However, what I do suggest is that thru your contract review and when you provide a quotation to your customer, you reference all of the required standards/specs with the latest revision that you have. If the customer comes back to you 5 years later and says you used the wrong level, you have some basis for an argument, i.e. they accepted your quotation which provided all of the relevant details. It is not fool proof but it's not bad either. But I am sorry, there is no easy way to check the latest revison of specifications/Standards.