What detail should I be looking for in Leonard in Memento to determine the plot?

You need keys. We need to help Teddy to find his keys! :-)

Joke aside, you should give attention to what Teddy says directly before the linked video.

Teddy: "So you lie to yourself to be happy."

and later:

Teddy: "Yeah I guess I can only make you remember the things you want to be true. Like Jimmy down there."

Leonard: "He's not the right guy."

Teddy: "He was to you. You got your revenge. Enjoy it while you still remember."

and sometimes later (that's in the linked video) Leonard is thinking:

"Do I lie to myself to be happy? In your case, Teddy, yes I will."

as he is writing down Teddy's license plate to form a new clue tattoo.

Also take notice of the memory of his wife in the scene where he is driving to the tattoo parlor--especially note the chest.

On his chest is tattoed "I've done it." while is wife is alive.

So, what do we make out of all this?

Everyone is using Leonard - including himself. He fudges his informations to stay on a constant path of Revenge. That is the thing that makes him happy. Probably it's because he killed his wife with an overdose of insulin, a story he attributed to Sammy Jenkins. He can't stand that, he needs to see "Johnny G." as the guilty for the death of his wife, although she survived the assault and rape (as his memory in the car-drive-scene shows).