If you have capacity you can make a decision, even if it is unwise. Assessing capacity includes:

1. Can the person understand the decision to be made and the information needed to make it? This includes the purpose and the consequences of the proposed action, or of refusing.
2. Can they retain the information for long enough to make a decision, a few minutes at least.
3. Can they use that information to make a decision, can they weigh-up the pros and cons of making the decision? This can be difficult to judge, but try to assess if there is evidence of logical reasoning?
4. Can they communicate that decision? If a person cannot communicate their decision at all, they lack capacity, but all efforts to help the person communicate their decision should be made, before deciding they cannot.