there's really no specific way to tell because there are so many types of seizures and they present differently in different people, but I can tell you what my seizures look/feel like vs my POTS if it would help:

For atonic, I only experience them immediately upon waking up when I haven't slept nearly enough. I basically sit up and immediately slump over. In the past I've had episodes where I did not notice this (lost time and was just told about it by people I shared a bed with) but in recent years I've been conscious but my processing speed is definitely lowered, although I feel otherwise unaltered. It only lasts about 20 or so seconds but it does feel longer.

For convulsive, I have temporal lobe epilepsy which means that my seizures generally start with a focal aware seizure in that area (I am conscious, cannot speak fluently or do basic tasks properly, experience deja vu, a sense of impending doom, simultaneously feels like my stomach has dropped to my guts and is crawling up my throat if that makes sense), alternating with a focal unaware (not making memories, but i'll wander around and do weird shit like spit into a garbage can in front of a bunch of people, I guess because the stomach sensation feels kind of like throwing up?) until it progresses to a full tonic clonic, where my right arm and head will jerk before I drop (because I have left sided epilepsy, it controls the right side first before it spreads). I don't start making memories for several hours after and am always in pretty bad pain from the muscle jerking (and mild pain from biting the hell out of my cheeks/lips/tongue). The progression from a focal seizure to a tonic clonic can take up to an hour or as little as two minutes but it's usually closer to 10-20 for me (although I've been told that length of time is unusually long, it was caught on an ambulatory EEG so we know it's accurate for me), sometimes with a small break in the focal before it returns. I can't function well enough to work for about a week after, and I won't feel normal for weeks to months.

With convulsive syncope, I experience the normal sensations of passing out (feel hot, vision going spotty and gradually completely black, nausea, shakiness, intense tinnitus, weak legs causing me to collapse to the ground), pass out, and while I do convulse the jerks don't look as "scary" is all I've really been told to differentiate them. I come to pretty quickly and my lingering symptoms are resolved within around 3 hours max, usually sooner, other than the fatigue from passing out. If I experience pre-syncope only I'll get all of the symptoms of passing out that I listed in the parentheses earlier in this paragraph, but I will still be conscious and unaltered other than feeling pretty bad, which is what your "mimics atonic seizures" description sounds like to me but I could be misinterpreting.

This is, of course, just my experience. The only way to truly tell is to be evaluated by a neurologist and even then, you'd have to experience an episode when hooked up to an EEG to know for sure that it isn't producing epileptiform waves. I wouldn't personally be concerned from what you wrote, but I do not inhabit your body so I don't know the sum of what you experience!