Three years ago I replaced both thermostats-just to be on the safe side, after all, they don't last forever...the thermostat that is up against the head (smaller of the two) the gasket broke down and started leaking two months ago, the round white gasket was an OEM part as the new thermostats. You can see the coolant on the top side of the tranny...watching your fluid in the overflow, it started needed to be topped off...this "new" gasket, once you take it off you will see that it is "crumbling" and it compressed somewhat too.
How you could tell when you buy a new set? I have no idea, they do look like the original, but like I said "they don't hold up".
2007 2.0 gas Caliber, CVT, 233xxx miles