I give a recording ten seconds to tell me who it is. If I still do not know who is speaking when the ten seconds are up, I hang up. Ten seconds is generous, because a legitimate sender has said its name by then and wants you to know it. A pharmacy says the pharmacy's name and a school says the school's. A message that opens with “this is an important message about your account” has told you nothing and has made you feel something, and the vagueness is deliberate, since a vague message fits everyone it is sent to. That missing name is the foundation for every pitch in how to tell insurance, solar, loan and health robocalls apart, where the hook changes and the blank where the name should be does not.
The sound of a script
Pay attention to rhythm. A recording usually has a steady pace, even emphasis and no breathing, and it may leave a small pause where it expects you to react. Some use synthesized voices good enough to fool you for a few seconds, so I would not trust voice quality alone, because modern text-to-speech can sound natural and old recordings can sound human. A more reliable tell is behavior: the voice does not change when you talk. Say “hold on” and if the message carries on without a flicker, it is a recording, which is the basic test in what a robocall is.
The opening line says a lot too. “Hello, can you hear me?” is trying to get a recorded yes, or to check that you are a live person. “This is your final notice” creates urgency with no detail. “Press 1 to speak with an agent” asks you to confirm you are there, and the risk in that key press is the subject of should you press 1 or 9 to get off the list. “Hi, it's Sam from your clinic about your appointment” at least names a context you can check, though you should still verify it by calling the clinic yourself.
Background sound is a smaller clue. A call center often has murmur and keyboard noise behind a live agent, while a recording tends to have either clean silence or looped hiss. A voicemail with background music is common among junk drops, as the guide to ringless voicemail describes. None of this is proof, only a nudge. Length matters in the same loose way. A very short message with a number to call back is the usual bait behind the missed-call and one-ring tricks, while a long one with a full pitch is a classic sales drop. A real notice sits in between: it says who, why and what you can do, in under half a minute. If a message goes on and on without telling you who it is from, you are listening to a script designed to keep you on the line, and the right response is to stop listening.
Names and mistakes deserve less weight than people give them. A message that says your first name or mentions your town has usually been filled from a list or guessed from the number, and local references are cheap, since a campaign can pick a different town name for each area code it dials. I treat a personal-sounding opening as a mild warning, not a comfort, especially when the rest of the message is vague. Awkward phrasing or a mispronounced name can mean a poor script, but plenty of legitimate automated systems mispronounce names and plenty of scam recordings sound polished. Judge by what the message wants, not by how it sounds.
What it asks you to do
The red flags are requests. Any demand to press a key you did not expect, say yes, confirm a code, read out a card number, or pay by gift card or wire should end the call, and a request for payment by gift card is a scam signal on its own for nearly any caller. The danger of a spoken yes comes up again in why a call goes silent or clicks before someone speaks. One sign proves nothing. Two or three together mean you can hang up without feeling rude, and the same message arriving from different numbers, as in why the same robocall keeps coming from different numbers, is a good reason not to wait for a third.
Some recordings are legitimate: a school alert, a pharmacy notice, an appointment reminder. If you recognize the sender, check on your own by calling the number you already have, not the one in the message. If the notice is truly urgent, the sender will reach you another way. You can also look at where the number was issued with the number check. It gives only area code facts, but a code from a distant time zone at an odd hour is one more small signal.
What I save when something sounds odd is boring and small: the time, the number shown and the first sentence I heard, in a note or on paper. If your phone shows a voicemail, keep it until you have reported it. You do not need a recording of a call itself, and recording laws differ from state to state, so I would not try to record a stranger without checking the rules. A short written note is enough for a report at reportfraud.ftc.gov, and a few dozen specific notes show the agencies which pitches are running better than one long story does. I have never regretted a short note.