I always check the opening moves against opening tables
This is the first thing I do after a game played. If I am sure I know the opening, I may not do this. But if I was even a bit uncertain, I do. If I recognize the moves, I step through them fast. However, I do step through them so I am sure to recognize what was the "right or popular way" to open. This is part of the positive feedback enhancing learning.
Even if I may have done the first couple of moves right according to opening tables, there will likely be a point where games have been played before where I deviated. Sometimes I do want to deviate, but sometimes I find I should not have done that.
If there is a move which is unknown to me, I check what is recommended as my response, and I may check what is then the next expected response.
Check the recommended opening move by looking at played games
I use Chessbase Powerbook 2014 as my main source, as long as I follow a line which is well played. I simply look at the position, and then I see what is the move with best percentage for my color, and then I go forward to next position and so on. If I check sidelines, it is the same method, just make the sideline move and see what is the expected response. This makes it a very quick follow up method.
If I am white, it is easy, all percentages are expressed in white winning percentage, and I look for the ones with the highest percentage. If I am black, it is just as easy but how to read the figures depend on which program I use. In Friz using Powerbook, I need to look for the ones with the lowest winning percentage since the percentages are expressed from white. In chess assistant from ChessOK, the percentages are given from blacks perspective when you look at a move for black.
I always check what move is used by the highest ranked players. If the move has high rank, many users playing it, and it feels right for me in the situation, then that will be my targeted move.
If the highest percentage is used by only few players, or only low ranks, then I may be very doubtful on using it. One thing I might then do is to look at the same position in ChessOK Chess assistant, using hugebase (which contains a lot more games, and is similar to Chessbase Mega database). If still doubtful, I will let the chess engine generate moves so I can see how the games is supposed to move. I then analyze to a deeper depth than normal, which combined with being in the opening means it takes a lot longer time than normal. So it is rare I do it. An alternative is to find something around the opening on Internet.
All in all, I get hints on better or at least alternative opening moves. And I may understand why moves I have liked are not good.