You can't. Many people on GAG really like to show off what they think they know and pretend to be Sherlock Holmes on GAG. They're so busy trying to impress random strangers with their arrogant speculations that they don't care if they're wasting someone's time who's sincerely in need of help. I find that to be tacky.
It's tough to tell, but I think if the person who has asked the question is a troller then the person will deliberately reply to your opinion in such a way that will annoy you, irritate you, will say irrelevant things and will also get to you at personal level, then I think you can come to know that the question was a troll.