Weaknesses are defined by moves that you make that allow your opponent to capatalize whether it be in the opening, middlegame or endgame. Weaknesses are made in various amount of way, you could have a backward pawn, isslated pawn, destroyed king castled position, and much more. Weaknesses are also something that can be made by only you, you are the judfer and the monitorer of your position, your goal is to make it better and not make inferior moves, and for this game it means that from whoever can make the best moves and the least amount of weaknesses will win the game. So this topic about knowing what weaknesses are and how to avoid them is very crucial so you are definitley on the right path @thakururaman.
To detect weakness you will have to understand the position and how your opponent can use your weaknesses if you have any that is made, to prevent them from using them ex.(outpost square, bishop slicing to prevent the king from caslting because the lack of a lightsquared or darksquared bishop, etc...)
Truly the goal of the game of chess is to win without making weaknesses however this is impossible, there are certain moves that will weaken a postion it is natural, such as a pawn push to e4 or d4, these moves are not considered weakening however they left the control of 2 sqaures if for e4 it left the control of d3 and f3, and for d4 it would leave the control of c3 and e3, however these are not considered weaknesses because the moves are made for a common goal of controlling the center, opening development, and gains more space so you can work with more variables on the board in terms of pawn and piece play.
Now of course e4 and d4 are the best moves in a position in the beginning typically, however depending on the position and what it gives, depends on what is played, and this is where understanding comes in. Understanding tells what to do in a position and what could be the positivies but also the drawbacks of a specific move or a specific set of moves. This is the beginning of understanding positions and positional understanding. Firstly you have to see if a move will weaken your position, if so it probably isn't a good move but then again what is the position calling for, is it possible? Are you in the opening, middlegame, or endgame? And if you did this move would it have long-term positional affect on your position that is positive or negative?
So simply you can only understand something by gaining the knowledge, which turns into experience in which turns into understanding. I think that you will need to play more and play more moves that aiming to improve the position rather than wild and dubious move as if you do this plenty of craziness can come on the board, firstly losing tempo(in which is the main killer for many positions, especially for beginners), secondly weak positions and eventually the superior position will win, thirdly if you are making weakening moves you are only giving your opponent room for tactics and ideas that would normally wouldn't even be possible, but that is what weaknesses can do.
So my advice for you is to: Play slow classical chess(20-30 minutes) and evaluate and monitor your position, check to see if you have weaknesses and try to avoid making them. More importantly improve your position, this is key, if you don't improve your position you cannot do anything with it and you lose tempi so you must constantly be looking to improve upon it, and only then strike with superior forces for a balanced or imbalance of a game, your opponent should do the same and if not then you are in a better position just by activity. Also you must know when to look for strategical motifs and for tactics, and there is a fine line between them, usually tactics can be spotted anywhere from the opening to the Endgame, same is for positional play as it makes up chess. What you need to do is know when to play certain moves and do certain motifs as this is what understanding is, knowing what to do, when to do and how to do it, this is also comes with intuiton later on.
To tell you the truth detecting weaknesses is just something you will have to learn and know it is not so easily said to be understood because there is no active learning involved with this. Even for what I am writing it is passive learning because you see what I am writing yet you are not learning to much because your not playing and experiencing it. So truly if you want to learn you will just have to play more and get more experience so you just know and then you can understand clearer.
Trust me there is no lie when people say that failure is the best teacher, and you will learn however you won't truly learn until you experience how to make a difference in your play when you are playing.
Honestly though I don't know anything of your game but I was just giving general knowledge of what can do to help you spot weaknesses but this question is very broad and there is no right or wrong answer there is only an answer that seems most helpful to you @thakururaman and hopefully I helped you and didn't make this process more confusing for you.
Peace and good luck on your journey @thankururaman.
