It's the same on every forum. There are signs, usually. Here is a prime example, the person types out the assignment word for word (it looks like an assignment). Here's a biggy, when in doubt, ask the person. Either the person will tell you the truth or lie, and whether you choose to provide the answers or to ignore the post is up to you. That's your responsibility and yours alone. Moderators might come along and close the thread or delete the answers, or not, or some other person might come along and provide answers to a posting that you suspect is an assignment. (Are the answers even correct? Giving people the wrong information is part of the issue too.)
Answer what you want.