"They have got good players all the way through the squad, but I think the two forwards are strong," said Beckham after playing 90 minutes in Milan's 1-0 win over Cagliari.
"They are quick; they can hold the ball well and they are not frightened – so if you can control them, then you have got a chance.
"But Man Utd have also got players that can win games, and their defence is exceptional at the minute."
United will be in buoyant mood when they face Jose Mourinho's Inter, after opening up a seven-point lead at the top of the Barclays Premier League.
But their opponents lead Serie A by nine points and will be equally focused on this game, if not more so after failing to go beyond the quarter-finals in the last three years.
"United are favourites in my eyes, but I'm biased," said Beckham. "I think they are because of the strength of their squad and the way they have been playing recently. They are on a run where they look unbeatable, and I hope that continues.
"As a Man Utd fan, I'm on their side – and also as a Milan player. I'll be coming to the game and I'm sure I'll speak to some of the players and the manager. I'm looking forward to it because they are two great teams, two great managers and they have got some great players in both teams – so it's going to be a great game."
Beckham will be back at the San Siro again on Thursday night for the third time in five days, to appear for the Rossoneri in their Uefa Cup clash with Werder Bremen.
Victory in that game will be essential in dissipating the pressure which is mounting on Milan coach Carlo Ancelotti, and Beckham will do all he can to ensure he remains in charge.
"I am fortunate in that I had a great coach at United and I have got a great one here too with Ancelotti," he said. "He is very good with the players and enjoys huge levels of respect. I hope I can still play under him for a long time."