Although even Masters can play very badly at times, especially against other Masters, and apart from ELO ratings, how to you get the feeling that you are definitely of "Master" strength, and no longer of "Expert" strength?
If a player is truly stronger than you independent of his current form, he is able to outplay you even if you don't overlook a single important tactic. If he is in bad form he will probably overlook tactics himself and let you back into the game, but you will still be outplayed in the stretches between his little blunders.
Once you don't have this feeling anymore of drifting into trouble without knowing why when playing against master players, and once you are able to just outplay weaker players by hand, you can be pretty sure that you have reached a master's level of understanding. Your Elo should follow suit.
You get a sense you are master strength when you are playing over GM games and can actually understand the reason behind the majority of their moves as well as being able to predict a large number of them. When playing against master level players you don't feel outclassed and feel like they aren't doing anything you wouldnt expect. Lastly you don't have any huge weaknesses in your game. You might be weaker in some areas but overall your opening, middlegame, and endgame are all mostly strong.
Games are lost in two ways; by outright errors, which lose immediately, and by "slack" (suboptimal) moves, which cause your position to erode over time.
Most "good" games (the ones free of obvious errors) are races. E.g. races to attack each others' kings or to queen your pawns. A better player, by definition, will win most races. A weaker player will lose a move here and there, play a "good" (solid) game, and still lose.
You are a master when you can keep up (win some races, draw most others) with masters. You also be able to edge out experts in most races, except on a bad day.