For me it way relatively easy I guess, since my stories have pretty clear inspirations in the sense that I can clearly identify 'oh, I want to make something with XYZ vibes'. So my target audience would be the same as the target audience of my inspirations 
One thing though is I don't think it's useful to think of target audience in terms of age/gender etc. Rather, think genres and special interests. If someone posted a panel of your comic onto a subreddit, what community is it most likely to be?
Don't forget you yourself are also part of a demographic. What communities do you frequent? What tropes/inside jokes are they familiar with? You probably have a few insider perspectives already; tap into that!