The test ids ARB1 and ARB2 indicate (s) that two different samples were used, rather than representing different test methods. My colleague is of the view that the subject "test ids ARB1 and ARB2" is plural, and hence the verb would be in it's plural form (without an s) "indicate".
Yes, it can depend on the style guide you're using, but since you're clearly not using a style guide, the plural of cat is cats, and the plural of ID is IDs. Simple as that. There is no reason to even consider an apostrophe. It conveys no additional information that the simple -s does not. As to "how to tell", what do you mean? They sound completely identical in speech. Lastly, there is no ...
Most dictionaries state "ID"/"I.D." as an abbreviation for "identification" rather than "identity", so it's no surprise that Etymonline directs I.D. "specifically" to "identification". E.g. From Collins Dictionary: ID in American English (ˈaɪˈdi) Informal NOUN Word forms: plural ID's or IDs identification US a card (ID card) or document, as a birth certificate, that serves to identify a ...
What about in non-bar contexts? In San Francisco, the Sundance Kabuki is an upscale movie theater that serves alcohol for some screens, and they have a person checking IDs at a checkpoint. The word "bouncer" just sounds wrong to me in that context, but the only alternative I can come up with is "ID checker" (which sounds equally odd to me).
A security guard wishes to address multiple people respectfully at once. Does he say: Sorry, sir, but this ID is invalid. or Sorry, sirs, but your IDs are invalid. or ...