Is it ok to comment an answer to the OP because you can't test it, and then you answer if it works? Or should you just answer?
I'm a bit more relaxed about this than Jacob, mostly because I agree with the last sentence. Comment answers are a real pain because people trip over themselves in politeness to not transcribe it out into an answer.
If you're just asking for output, it's a comment.
If you have reason to think something will work, it's an answer. Explain that reason, what you think is wrong, why you think your solution will fix it, and you probably have a good answer.
If you're doing both (eg you're guessing that it's a specific type of video card based on the symptoms and have a solution for that), explain all that and it's an answer.
It doesn't really matter if it's wrong or you made the wrong assumption. You can edit. We can edit. You can delete. We can delete.
Obviously if there's huge uncertainty, you could save yourself a lot of effort by asking the OP to improve their question with specific detail before plunging in, but more than a few times I've ridden an assumption through to an accepted answer.
If you are unsure if your answer works, often it is because the question lacks important information or isn't clear on all points. To clarify the question or request additional information, that would be the goal of a comment in general.
If you are unsure however if your answer is correct, I'd do some more research to find out the validity of your answer.
Having said that, it happens to all of us from time to time, but if you do, make sure to keep an eye on the question to convert the comment into an answer if it turns out to be working; comment-answers are a pain.
I believe it's okay, I have done it like since forever. But the point is:
convert your comments later to an answer if it works.
I have been in many situations where something comes to mind first on reading a question, but it's a general "something" like permission issues or cache or anything else.
I would first comment on that and ask for more information. If it turns out I am right, I will convert it to an answer. If not, I just leave the comments there.
This was a permissions issue:
Where is .deb package location after I run apt dist-upgrade on Ubuntu 16.04 LTS?
Download server settings:
My Ubuntu 14.04 doesn't update anymore