That's too broad a question, some examples of you misjudging the position might help.
how to know if a position is draw

Well one thing you don't do is offer a draw when the only one who can lose is your opponent. You at least have a draw (in fact, you're much better here), he's the one who should fight for it. It's not easy at all for black.
If it's not clear to you why white is much better here, it's just another reason why you should play on. That's how you gain experience and learn to evaluate the position and build a plan.
Here 1.f3 comes to mind, then black is forced to go 1...Ra8 to save the pawn, you push b4 anyway (black can't take on b4 or he loses the rook), and then b5 and you have a passer. White's a-pawn is impossible to hold on to, so you'll end up with a passer and two extra pawns, that's piece of cake from there. If it's still not clear to you why at that point it's an easy win, I'd recommend studying basic endgames.

Of course it is a winning position for white . Now white should play f3 to protect the g4 pawn ,and then the rook will capture the a5 pawn inevitably . So white is up two pawns in this endgame , white would win this game soon .

I've gone into some endgames where I offered a draw when I was winning, or not offering a draw when it was literally impossible to make any progress. I've done some researches on my own but I still can't find the answer I need. So how to tell if a position is draw? And how to avoid blundering a draw when winning?
You may learn more theoretical knowledge in endgame ,such as most of up-two-pawns position will lead to a winning position in endgame , that is useful in that position you have occured .
At a glance, I can't tell if this position is a win or a draw, although I strongly suspect that White is simply winning. But the OP's problem is not with this position, or with any lack of endgame expertise.
His problem is with the his competitive spirit. What he is basically saying is "Well, I'm a pawn up, and I probably have good winning chances. But my opponent might be able to create some threats and if I fail to play accurately he might win his material back. And if I make even more mistakes I might lose! I'd better offer a draw!" Who wants to play chess that way?
... His problem is with the his competitive spirit .....
Right! The thrill of the battle and the unknown future is what you play chess for. Just let it happen!

At a glance, I can't tell if this position is a win or a draw, although I strongly suspect that White is simply winning. But the OP's problem is not with this position, or with any lack of endgame expertise.
His problem is with the his competitive spirit. What he is basically saying is "Well, I'm a pawn up, and I probably have good winning chances. But my opponent might be able to create some threats and if I fail to play accurately he might win his material back. And if I make even more mistakes I might lose! I'd better offer a draw!" Who wants to play chess that way?
True, this is also an issue I am working on. Just trying to avoid overthinking and play good chess.

You have more than just a pawn advantage. Your knight is slightly better than black's bishop, and your king is much more active. (King activity is important in the endgame.)
To the OP: I didn't mean to sound too critical. I have also had to work on becoming less risk-averse. In the past, I have accepted draws in positions where I had some rea winning chances, particularly in the last round of a tournament. But in the car driving home, I've always bitterly regretted that choice

Though it is hard to say, especially without seeing a specific position, in endgames there are two key things that you can look at that might help. One, being the activity of the pieces. Is your opponents king able to maneuver around the center freely or is it instead pinned back on the back rank. Are your rooks attacking the pawns from behind or are they instead passively protecting the pawns. Another thing that you can look at is how advanced your pawns are in comparison to your opponents. Likely, if only one side has a passed pawn that can advance, they are probably winning.
There are many more signals that you can look for but, if you notice any of the above, you are likely going to win the game or are at least in no danger of losing.

In this position its a bit more complicated because you are playing on the queen side and your opponent is attacking more on the king side so it is a race. But, depending on whose move it is, if you notice that once the a5 pawn falls you can easily attack the d6 pawn and you have 3 passed pawns vs one (potentially), you might realize that you are the one playing for the win.
In general, if you are ever up material in the endgame you should keep playing.
I've gone into some endgames where I offered a draw when I was winning, or not offering a draw when it was literally impossible to make any progress. I've done some researches on my own but I still can't find the answer I need. So how to tell if a position is draw? And how to avoid blundering a draw when winning?