Gents, pretty much I have found in my 116 journey, this is what I do, and I am sure other members will have other tricks too.
- Get part number - for this you will need some form of EPC access
- Next I google the part # and check for hits.
- If you see very little hits, or links to obscure Russian websites, this is a bad sign
- First prize is you see a link to an ebay listing, still active at a good price then pull the trigger, no brainer, especially if it is NOS. I very rarely buy used parts, just not worth it unless you can confidently rebuild
- If you want to check if still available ex Benz, I only use the Classic Centre. Pricey but this serves 2 purposes. You know if it is till available new, and then of course you know how much you are going to have to pull your pants down for

Cheers