Hang in there. This is the hardest part of support work. The fact that you think about it in these terms puts you ahead of many in the industry.
I have worked with many challenging clients over a number of years. Consent comes down to how confident we can be that any communication from the client reasonably represents their agreement or disagreement.
Concrete, unambiguous gestures increase our confidence in the clients consent. If the client is verbal, get a verbal response, and a gesture response (like a 'thumbs up') as well. If non-verbal, work on two concise gestures each for a 'yes' and 'no'. Make sure they are meaningful and unambiguous.
Think of it like Two-Factor Authentication. If you require consent, make sure you get a 'yes', and 'thumbs up' before proceeding. Break it down for them. Ask once for a verbal resonse ('Would you like to go to the movies?'). Then again for the gesture ('Can you show me if you would like to go?').
When you record your notes for the day, you can better justify how you determined client consent. Talk to your Allied Health Team. If they are decent they will be on board with something like this. A system like this takes time, so be patient.
Be Well my friend and Strive for the Good
RE: How Do You Give Consent When You Can’t?