How to know if you are tremolo picking in time? Let's say you are going around 16th notes 160bpm and you are tremolo picking power chords. How do people actually know they went up and down 4 times in one metronome click? It's fast so you can't count it out. Do people just go by feel?
That tempo is, arguably, too fast to count (is a bit above 10 notes per second). But you don't need to literally count (out loud or otherwise) to hear whether you played 2, 3, 4, or more notes... you need to train your ear on what groupings of 2, 3, 4, or more notes sound like. You need to become familiar with these rhythms at a slower tempo, a tempo where you can count, and so familiar that you don't need to count the rhythm at all to identify or accurately play it. This comes through repetition. Most metronome apps will have the option to click eighth or sixteenth notes at you, which may be helpful for this. Then you can bring the tempo up.
It may also benefit you to practice rhythms without distortion, since it can obscure the note attacks and make it harder to hear rhythm.
Four subdivisions of 160 is definitely possible to keep track of with the ear and with the body. It's certainly not impossible even to count out loud, though it becomes a bit silly and slurred into "WututhrifoWututhrifo." There does come a point where things start to blur together, often right around the point also where our body just can't make the motions any smaller, but it might be somewhere more like 8 subdivisions of 125 or so.
And yes, the brain does think of those four notes as a unit, a pattern. You know it's 4 because 3 would be triplets and 5 would be... uh, "Take Five." You don't really have to ask yourself "Did I play 3 or 5 notes," because the four-note subdivision is itself a pattern.
So what do you do? Get used to it at a slower speed, then gradually increase. Can you do 16th notes at 100? 80? Start at a speed at which you're rock-solid, every note is in control and is the same length. Then add 5 bpm, then another 5; at some point you'll get into the gray area between "no sweat" and "impossible," and that's where you learn. Keep pushing that line every day. By the time you get up to 160, no question, your ear and your hand will be able to easily keep track of the four notes.