The Dutchman, who won the league title with Barca in the 1998-99 season, believes his former team's sense of identity can help them to come out on top in the decisive battle.
Real Madrid are three points clear of Barcelona with a game in hand on their rivals, but Barcelona would go top with a victory thanks to the head-to-head record between the sides.
De Boer highlighted the attacking talent that will line up for Real Madrid and Barcelona at Santiago Bernabeu on Sunday as holding the key to the match.
"The top players in the world are playing [in Spain]," de Boer told Omnisport.
"The Spanish clubs in the last few years have dominated the Champions League, especially with the likes of [Lionel] Messi and [Cristiano] Ronaldo, who are the best players in the world in both teams.
"It's about who is in good shape. Right now it's quite equal. Real Madrid are quite stable this year in every position. They can score from Ronaldo, [Sergio] Ramos, [Alvaro] Morata and [Gareth] Bale .They have so many stars that you can just put 11 on the pitch and they can play with each other.
"Barcelona in my eyes are more of a team. They have an idea of how they want to play. Neymar is lifting up his form, Messi is sharp again with [Luis] Suarez. I think they're both in good shape, so it's hard to tell who will win."
De Boer believes Neymar is stepping up in class, so he will be ready to take over Messi's mantle when the Argentina international leaves Barcelona.
"He's taking the leadership at Brazil," de Boer added. "I see he's becoming more and more important for Barcelona. He is one of the best players there is on this planet.
"When he gets a little bit more mature he will fill the shoes of Messi. Neymar is getting there and I don't know whether he'll be as good as Messi but he's certainly trying."