Several ways. Obviously you can simply check the popularity of certain hashtags yourself by searching them. Secondly, theres also hashtag suggestions, so if i were to write "#tot" and #totkcontraption is the first one, its probably the most popular one. Lastly, you just use 2-3 hashtags. So #totk#tearsofthekingdom#totkvehicles for example.
Its not a perfect system that will sort every post perfectly, but its an open and flexible system.