Various strands of evidence point to that: how she was regarded by giants on the global stage, such as Obama; how she was regarded by those who came into contact with her who weren’t Royalists; the loyalty she inspired in those who directly served her; what she did (and didn’t do) with the power she had; her sense of duty from which she never wavered; the impact she had upon ordinary people she came into contact with; the way she dealt with the Good Friday Agreement and put her personal losses to one side to secure peace. These are all examples that point to her character.
I get why people don’t want a monarchy, in fact I think it’s time for a debate around this subject and at the very least it should be hugely scaled down, but just because you can’t choose your monarch doesn’t mean they aren’t capable of being a great leader, which is what she undoubtedly was. The narrative of those she came into contact with is too congruous with that for it not to be the case.