How to refuse alcohol to an obvious adult without ID

Hello, I am a new server and not of age to be drinking, but I am able to serve alcohol. At my restaurant I was never given proper training, as I was a host before and they would just make me serve when they needed help and now they gave me the server position. I feel very awkward refusing alcohol to an adult who is obviously over 21 but they do not have an ID. What is the best way to say I can't serve them, especially if other adults in their group are ordering alcohol? And do you ID everyone despite them looking 40+? Do you ID regulars every time if they don't look under 40?