You can't know - at least, not in any objective way. The real question isn't, "What am I good at?" but rather: What can I let people (including myself) do?
What do you know for a fact that you could help people do? Can you help people feel heard and understood? Can you help people save time, energy, money? Can you help people feel more secure? Can you help people develop a certain skill? Can you help people entertain themselves? Can you help people feel wanted, desired?
Chances are, if you can help someone do something, then you can identify what you're good at. Not necessarily the other way around.