I've previously heard the suggested language of 'Am I being detained or am I free to go?', which seems good enough to make the situation a lot less ambiguous.
Hence the importance of keeping major legal issues as simple as the Founding Fathers instantiated them in the Constitution and Bill Of Rights: the common man is presumably incapable[1] of keeping up with voluminous manifestations of nuances of legislative and case law. If the police aren't expected to keep up with it all, in no way should prospective detainees (i.e.: all of us) be as well.
But, alas, the legal system being what it is, that's why those of us smart enough to try to keep up with such matters must do so: protect, at least, our own skins.
[1] - Not in an insulting way; there's a reason we pay lawyers big bucks to know such things.
2. No.
1. Okay, I'm going to walk away now.
2. You can't do that. I'm asking you questions and you are not free to leave.
1. But you just said I am not under arrest. Are you arresting me?
2. No.
1. Okay, I'm going to walk away now.
... [a few iterations later] ...
1 starts to walk away.
2 informs 1 that s/he is not free to go.
1, on the basis that 2 has said s/he is not under arrest, and tired of the back and forth, begins to walk away.
2 performs a physically or verbally threatening action to prevent 1 from leaving--e.g., blocking exit path, ordering to remain in place, etc.
1, on the basis that 2 has said s/he is not under arrest, and tired of the back and forth, continues to walk away or otherwise informs 2 s/he is going to continue to walk away (or ask 2 to leave).
... [several rapidly escalating iterations later ...]
1 attempts to walk away.
2 physically apprehends 1 in some fashion and orders 1 to stop.
1 continues to try to leave.
2 decides to arrest 1 and charge 1 with obstructing, opposing, and endeavoring to prevent (with or without force) an officer in the lawful discharge of his duty. 1 is at increased risk of physical harm from 2 in the course of being arrested as this is the kind of situation that appears many times in cases where an officer uses excessive force to apprehend a 'difficult' individual (whether s/he actually was being difficult or merely attempting to assert his/her rights and avoid being bothered by the officer).
1. Am I free to leave?
2. No.
OK, now you've made it clear you are under custodial interrogation (I suppose there are safety exceptions to that), Miranda is invoked according to the Orin Kerr, and you can keep your mouth shut beyond the requirement to give your name in some states.
There was a video a few months ago showing someone recording in-country immigration checkpoints (dozens of miles inland, checking IDs without cause in a very unconstitutional manner). The driver reiterated "am I free to go?" until he got a "yes". Chilling, and informative that "am I free to go?" is the ONLY thing you say/ask. If they say "no", lawyer up & shut up; if "yes", leave.
Only question remaining is at what point does the LEO's refusal to say "yes" or "no" constitute one or the other.
Furthermore, one could hope that your refusal to answer his questions with anything other than "Am I free to go?" would not be so easy to construe as this sort of silence signaling guilt.
Since your leaving could be spun as felony (in at least some jurisdictions) fleeing the police, you'd best not leave without their OK, explicit, or implicit by their leaving first.
Note: don't let yourself get startled by the police into running away. One of the things that reified all I've been reading about the modern police in the last decade or so was retiring and returning to my home town, and one day in 2007-8 a new, improved policeman played a game of chicken with my body and his car, that is, as soon as he noticed me, he veered it on a near collision course. I was an experienced enough pedestrian---which I think is unusual here---that I could tell his vehicle would miss me by a good foot or more and the mirror by at least half that, so I stared him down, and he then veered away. Based on my readings, I have have to assume he was trying to get me to run ("this crazy cop is trying to kill me!"), at which point the crime of fleeing the police could be claimed.
Nowhere am I advocating that. I'm only suggesting "Am I free to leave?" and silence in response to anything but an unequivocal "Yes" to the question. And noting their leaving is also an implicit Yes.
Now, depending on your reading of the situation and the cop(s), you could play games with an equivocal answer, i.e. follow with "OK, you are detaining me", and if they object, go back to the free to leave theme, then press them on deciding one or the other. But that's not what I think one should generally do.
Needlessly complicated? I suppose that's a matter of interpretation when you're on the receiving end of a power imbalance and are wanting to both go on your way and assert and protect your rights.
Yes, the shorter path is to ask if one is free to leave, and then respond accordingly. And yet still, it is not incomprehensible that an officer might respond with something more complex than a simple "No" when asked if you are free to leave. Say, something along the lines of, "Yes, as soon as you answer a couple questions." This is much closer to a "no", but still legally debatable (especially within the context of this decision) whether or not you are in a situation in which you may rightfully assert your rights.
For that matter, I suspect most will realize that they aren't going to get anywhere with someone who's knowledgeable and disciplined enough to only say "Am I free to leave?", and will move on for easier prey. Spending "hours" in such a game isn't going to generate the arrest statistics they need, they'd only make that sort of investment if it was a really serious felony. (Unless of course their reward was an internal one of successful harassment ... and this sounds very tedious.)
You have the right to remain silent, remember?
50 minutes long, but worth it.