Unless the POA specifically states that a health professional must decide whether they have lost capacity (it is an unusual clause - most do not have this), then the best person to decide whether they have capacity is the person who is with them most. - probably you.
Capacity is a slippery thing. You can have capacity for one thing, but not another and they can have capacity one day and not the next.
@MartinWL has a list of very useful pointers to help make the decision about capacity and I hope he will post them for you
I am flattered by the recommendation but don't personally deserve any credit as what I have posted before comes straight out of the Mental Capacity Act 2005. To ascertain whether a person has capacity for a specific decision:
1. Does he/she understand information given to him/her in relation to the decision? (e.g. a diagnosis, facts of the matter etc)
2. Can he/she remember that information for long enough to consider it and make a decision?
3. Can he/she weigh up the pros and cons of different courses of action?
4. Can he/she communicate the decision he/she makes?
A person has to be able to do all four things to have capacity. The person is entitled to make a stupid foolish decision if he/she so wishes, if he/she has capacity. Every decison has to be considered separately - just because a person can't understand the issues involved in, say, selling off their stately home in the country to pay for repair of their flat in Bayswater, does not mean that person lacks the capacity to decide what shoes to buy.
I had to use these tests myself in a difficult situation, when I sold my father's car against his wishes, after his driving licence was cancelled by DVLA. I reasoned that he had not understood his dementia diagnosis or why the doctor thought he was not fit to drive, because he essentially didn't believe there was anything wrong with him, and it was therefore impossible for him to pass tests 1 or 3. So I sold his car and endured the resulting fury. It is not laid down who exercises the wisdom of Soloman in these situations but as I had POA it unfortunately fell on my shoulders to do the dirty deed.