Can I check if a key is valid or fake/used without adding to my Steam Account?

No.

There is no capability to check Steam keys unfortunately.

Nope

You can check if it is valid trying it on account that already has the game, but this doesn't mean it's not been used already

Apparently (i.e. internet people have said it, but I've not tried myself) Steam Support are able to check if a key has been used, or if it is still good, what game it is for, etc. If it's important, you can try asking them. Good luck! :)