At a press conference before the Spanish Super Cup, Barcelona coach Xavi said that Wednesday's Clasico was the best chance to test Barcelona's current level.
In the Spanish Super Cup in Saudi Arabia, Barcelona and Real Madrid meet in the semi-finals, and the winner will compete for the title with the winner between Atletico Madrid and Athletic Bilbao.
Barca has made some progress under Xavi but is still suffering setbacks, such as last Saturday's 1-1 draw with Granada, and Xavi called on his players to use Wednesday's game as a starting point.
"This game is a huge opportunity to get to the final and could be a turning point for us," Xavi said on Tuesday.
"Real Madrid is the best team in Spain, the La Liga table shows that, but it's a different game. Clasicos are always unpredictable."
"Going to the final will represent a turning point. But whatever happens, we will be realistic. It's an important game and a huge test that will show what it will be like 2 months from now."
Real are at the top of La Liga, 17 points ahead of sixth-place Barca, but Xavi said it was a different game, a knockout.
Xavi stressed: "Clasicos are unpredictable, I've been through all types of clasicos. There are games where we are the favourites and lose, and there are games where we are underrated and win or draw at the Bernabeu. Contest."
"We will try to use our weapons and compete," he said.