From time to time I'm a new ham's first contact - usually on a repeater. If I want to send the new ham a QSL card, how should I distinguish a repeater contact from a non-repeater contact? Is there a standard?
I never saw a standard in the last 25 years. I usually put something in the comment like 'using repeater xxx'. Where the xxx is the callsign of the repeater.
I'd just add a note indicating the repeater frequency and call sign somewhere on the QSL card you send.
My XYL and I like to give ARRL First Contact awards to people we've made their first contact with.
Many times they're local people we know, so we just have the printed certificate sent to us. Then we give it to the person at our next encounter.
Nothing. You do not send QSL for repeater contacts, only for direct contacts.