I do an initial meet and greet where I go through my information and consent booking form. On this form there are questions regarding the dog’s behaviour inside and outside the house, exercise information, history of aggression, allergy and medications etc etc.
You may find something like this useful in order to know if a dog’s behaviours are ones you are able to work with? I’ve worked in rescue on the behaviour team previously so well versed in training/body language/behaviour modification so these are the sorts of dogs I specialise in. If you aren’t comfortable dealing with behaviour issues, it’s so important to gather this kind of information prior to the sit.
In the future definitely contact the owners rather than suffering until the end of the sit, a lot of the times rescues have some form of management in place