Most airlines these days offer online checkin, complete with digital boarding passes downloaded to your phone. However, on international flights, you're often -- but not always -- required to go to airport check-in anyway, so they can scan/sight/confirm your passport and any necessary visas. This is separate from the final ticket vs ID check at the gate, and if you try to skip it, frantic paging before boarding starts will ensue.
So simple question: How do I know whether I need to detour to get my passport checked?