This is the beginner in me, but HOW do you know you're on beat 1 and not beat 3 of 4/4? The reason for the question I don't see a difference between 4/4 and 2/4 on the accent of the beat the way pop music is played. So any downbeat becoming my yardstick for staying on time (altho as a beginner just staying on time with the beat and the steps is a problem in itself ). From this, 6 count steps vs. 8 count steps don't mess me up. Ignorant is bliss
It's a slightly more advanced skill/ability to hear the 1. Once you have it, you just hear it and in enough time will find it hard to describe what you're listening for -- you just hear it. I think (since I've been "just hearing" it for at least a couple years now) that it's not one specific thing, like a change in the beat, but rather the sound and the feel of all the instruments and the melody and rhythm that reveal the structure of the music and that give me the feeling of where the 1 is. After following the beat becomes automatic and what to do with the feet and hands becomes automatic, then you'll have time to take in the whole of the song.
Don't worry about it too much right now, but you can still start your training. As you're listening to the music, listen for changes: eg, the singer starts a new line, the instrumental interlude starts, a new solo instrument starts. You might want to start out doing this while you're not dancing, since you already have enough to think about. Listen for one of those changes and start counting the beats. After 8 beats, you should musical clues that it's another 1. Now, every 32 to 48 beats (4 to 6 8-beat minor phrases) a new major phrase will start. This could be when the singing starts or when the instrumental part starts, etc. You'll be able to hear that before you'll be able to hear each 1, so use that as a starting point.
Oh, and another clue as to when the 1 starts: right before it your teacher will say, "5, 6, 7, 8!".