De Gea was at fault for goals both in United's Community Shield match against Manchester City and in their Premier League clash against West Bromwich Albion last weekend.
And Bale, whose Spurs' side travel to Old Trafford on Monday night, hopes to take advantage of the Spaniard's current dip in form.
"If any keeper's having a bad time, you're going to test them and it's going to be no different," the 22-year-old told the Sunday Mirror. "We're going to go to Old Trafford and give our all and test their whole team."
Bale fully expects, however, that De Gea will eventually prove his worth.
"It's obviously going to be nerve-racking for him to go to United and be in the first team, but he showed at Atletico Madrid and with Spain that he's a world class keeper," he added.
Ferguson, meanwhile, believes De Gea will get better protection from referees when playing at Old Trafford.
"He coped with the crosses into the box at West Brom with no problem - he just didn't get any protection," Ferguson said.
"You saw the challenges - an elbow in the face and a boy went under him and he didn't get the free-kick.
"It's disappointing when you don't get the protection, but he's at home against Spurs on Monday and it won't be a problem.
"It's just away from home where he might get subjected to that."
Ferguson also dismissed fears over De Gea's lightweight frame.
"David will go through the same programme of core work as the rest of the players do when they come here," said the United boss.
"(Edwin) Van der Sar was a tall, skinny kid when he won the European Cup (with Ajax) in 1995. He was the same - and don't forget David is only 20.
"But I have no problems with him, none at all. We have never mentioned the criticism this week that David got after the West Brom game.
"He's fine, he's done his training - there's no problem."
Ferguson also revealed that Peter Schmeichel suffered similar problems to De Gea in his first games for the club in 1991.
"Schmeichel had a baptism of fire, too," added Ferguson. "He went to Wimbledon in his first couple of games with us and screamed like a pig.
"He got no protection. He had Vinnie Jones, Alan Cork and John Fashanu all piling on top of him and battering lumps out of him. David will be fine."
Source: Team Talk
Source: Team Talk