(a) Yes, the acceleration is positive if the graph of position vs. time is curving upward and negative if it is curving downward. It might be hard to measure how much the graph is curving upward or downward.
(b) No, because you can only determine the change in velocity from one time to another. You would need to know the velocity at some time .
(c) Again no, you would need an initial velocity and position - all you know is the change in velocity. Once you get the actual velocity, you still know only the change in position. I guess you could have the velocity at some time and the position at some (possibly different) time.
I hope this answers your questions. Post again if you're still having trouble.