Space is always in terms of how far the pawns are advanced. If I have pawns on the 4th rank on the queenside while yours are on the 3rd, then I have more QS space because my pieces have 3 ranks behind my pawns on which they can maneuver while yours will only have 2.
If both sides have pawns on the 4th rank then neither side has more space in that area. Obviously anytime you have a pawn (or pawns) on the 5th, you have more space in that area.
Maybe the OP thinks that "space" is space per se: barren and unoccupied. So remove all pieces and pawns in the Qside and he thinks that there is more space on the Qside. You can correct me if it that is not how you saw it.
Anyway, as wafflemaster said it, pawns identify wether you have space advantage or disadvantage.
I think it was Yasser Seirawan in one of his "winning chess" series that had a pretty good method for counting space. If I recall correctly, he counts each square that is either attacked or occupied in enemy territory (Rank 5+). If two different pieces are attacking the same square, it is counted twice.
I don't think it has a lot of scientific value, but it gives a pretty good idea comparing White's count vs. Black's.
Sometimes a space advantage turns out to be not, and is actually either just overextended with no piece support or the other side can force an exchange of material, usually benefiting the side with less space as that other king is closer to the pawns. Sometimes the player with less space has more flexibility, such as in the famous hedgehog setup.
I'll try Seirawan's method on a few of the positions that made me think of this.
I just looked up the Seirawan section in Play Winning Chess.
He counts only enemy squares that you control. This would not include squares that are occupied by one of your own pieces unless they also happen to be protected.
Simply observe the mobility of the pieces
the one with more space is usually happier on the board (unless they have a significant material disadvantage).
Many books point out that one side or the other has more space QS/center/KS, but often, when there are still several pawns/pieces on the board, it is hard for me to see it clearly.
Is there a correct way to check space other than eyeballing it?