UUUMMMMMM.........you can visually check if anything looks new. Like if it is a genuine honda pump. Or remove the timing cover and check the writing on the belt. No easy way to do it unless there is a sticker hidden somewhere.
There's no real guarantee.... a TB that was just replaced 20k miles ago should have readable writing still on it. If it's the original you will barely even be able to see the writing, or not at all.
Check the TB cover and see if the aluminum bolts have tooling marks on them, then pull the cover off (just the front one) and take a look at the belt, rotate it a full 360 and see if you can get a good eye on the writing.
Also check the Crank pulley, it's a biotch to get off so it will surely have tool marks on it, still, no guarantee the TB was changed but maybe a hint.