Hypothetical scenario. At the end of last semester, I got okay grades in my courses. Not great, but passable. Let's say that today I somehow come across a time machine; I'm not able or willing to help my past self cheat outright (test questions, homework solutions, etc.), but I can give him a little motivation; I go back in time to last August and tell him that if he doesn't get his act together, he will screw up bad and lose out on a job offer.
Of course, my past self has thought of this very same issue before. He is, after all, a savvy Worldbuilding reader. I am quickly able to convince past me that yes, I am indeed him six months into the future. But, here's the problem; past me is just as much a skeptic as present me is. How does he know that I didn't come back in time to lie to him? How will I know that past me will walk away and ultimately do nothing (or worse, get lazier) because he thinks I'm exaggerating or stretching the truth a bit?
So, my question is ultimately this: How could I go back in time to the recent past (under a year) and convince past me of some lie? Also, how could past me determine that present me is lying or exaggerating?