Mostly, this is for your own benefit. Learning is a constant process. Understanding what you've done right and what you can improve on will make future assignments easier.
Do you feel accomplished?
Even if it's just homework and you have to do it, it stills feels good to know you did a good job. Do you feel that you did the best you could? Or did you just do enough to get by?
Did you get the grade you thought you would?
Why or why not? Usually, your professor will make notes on your paper pointing out areas for improvement. If not, try talking to them and get their insight. Your professor may have a strict grading system that you need to adjust to or there may be an area you need to improve on that you were not aware of. Keep any "defaults" in mind and you'll do better next time.
Feel free to ask for feedback.
Did you just present in front of a group? If you ask, many people will offer constructive praise or criticism that you can use to improve your next presentation. You can also ask your professor for feedback. Never ask "how come I didn't get an A?" - it comes across as challenging and many professors will not respond well. Instead, ask "what can I do to get an A on the next paper?" Most professors will be happy to tell you what to improve on to get that A.
Is your work, your work?
Looking back, does your argument sound an awful lot like that article you read? Be careful of plagiarism. It's easier to accidentally do than most people realize.