How do you know if you have an advantage in a position.

At this low rating, I never quite seem to know who is better white or black. I just keep playing and there have been position when I thought I was behind which the computer says I was better, and vice versa.

Of course I am talking about equal material positions. Its not like I am a rook down and then think I wonder who is better.

Just how does one know if they are ahead or behind.

It's not clear if your question is asking about positional evaluation or about detecting the presence of tactics. There's a big difference since each evaluation involves a different set of attributes. For example, in today's puzzle (https://www.chess.com/forum/view/daily-puzzles/7-5-2017-diversion?page=1) I wouldn't have guessed there was a tactical win in there, but there was. When the computer gives a numerical evaluation it is not distinguishing between positional and tactical issues, so if you're asking how to come up with a combined evaluation like a computer then you're heading down a fruitless path for developing practical evaluation skills.

computer evaluation = positional factors + tactical factors = meaningless number for purposes of human guidance

To simplify it, a chess position comes down to i) material, ii) structure, and iii) initiative or activity.

Material is straightforward.  All other things being equal, if one side has an extra piece, that side is better.  Easy.  Things are rarely equal, though, and that’s where things become difficult.

Structure refers to the pawn structure.  Pawn placement determines piece effectiveness.  If your pawns are doubled, unconnected and far-advanced, then you have permanent weaknesses.  This is especially true if they are the pawns in front of your King.  You never want weaknesses around your King, obviously.

The last one is perhaps the most important – and the hardest to fully pin down.  It’s piece activity or the initiative.  In short, it is who is calling the shots.  If every move I make creates a threat, then I have the initiative.  If you are forced to defend, then you cannot create threats of your own.  If you cannot create threats, then it is very hard for you to win.  At lower levels, initiative is often more important than material; we’ve all seen amateurs up a piece or several pawns nevertheless blunder when under pressure.  That’s the power of the initiative.

When looking at a position, I gauge all these factors.  If we are equal in two of them but I have an advantage in remaining one, then I’m better.  If I have an advantage in two, I’m much better, and an advantage in all three is likely winning.  That’s the easy part.  The hard part is where I have an advantage, say in structure, but you have an advantage in activity, the initiative.  Who is better?  Dunno.  Every situation is unique, and often we say these positions are ‘dynamically equal’.  These are often the most interesting positions to watch and play.

The stronger you get, the more this process becomes natural.  GMs can do it intuitively in seconds, or so it seems.  It’s a tad simplified, but that’s more or less the process.

I don't think you have sufficient time to evaluate who is better off in bullet chess. You basically have to choose an attack and do it without hesitation, hoping you're better off.