What are signs of achieving Master strength? 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.