Is there a way to know if a student has seen a grade I have posted?
Probably not, but it may depend on what you mean.
Grades are visible in many locations, but even going to one of the pages that has grades on it does not mean that the student saw any one particular one. To check when the student last visited the grades page, or any other asset within Canvas, you can use the access report for a student. Still, there's no guarantee that they saw their grade, but if they visited the page after you submitted the grades, it's possible (maybe even probable?) that they did.
If you're looking to see if they read a submission (assignment) comment that you left, then there is no definitive way to know. Those are often sent to the students email or to the Canvas app as a notification, so the student might have read them without Canvas knowing about it.
If you're talking about a quiz, then the Canvas API will will return whether or not they have looked at the quiz after taking it. The description in the documentation says "Whether the student has viewed their results to the quiz." I'm pretty sure that means that they looked at the quiz results, not just seen the score from somewhere else.
If you see your students face to face, the best way may be to ask. Don't assume that the students see the submission comments. I once had a very frustrating semester where a student kept ignoring all of the comments I left under submission comments. About 14 weeks into the 16 week semester, she finally discovered that I had been writing her with suggestions and things to do differently all semester long.
Kind of. You can see what pages a student has viewed by viewing their Access Report:
Which pages did my student view?
Retrieving data ...