The 'Undo It' singer, who married Fisher in June 2010, revealed to Glamour that Fisher has been totally honest with her since they began dating years ago.
"Any guy I've ever dated, there would always be that doubt - 'He didn't answer his phone. What's he doing? It's late'," Underwood recalled. "I'm not the crazy-girlfriend type, but there was a little voice in the back of my head.
"I don't have to worry about Mike. I don't think twice. He's going out? I don't care; it doesn't bother me. I don't bat an eye."
Underwood also suggested that her and hockey star Fisher's romance has lasted because they both know how to deal with fame.
"In Canada he was a big celebrity, so he knows what it's like living under a microscope. And he travels a lot, which is good for both of us," the musician noted.
"We're both independent people, so we can't have somebody around 24 hours a day, seven days a week. The drawback is that we can go weeks without seeing each other."
Underwood recently told Digital Spy that she would never judge a talent show like The X Factor.
"Judging - that would be a really tough job to have because there would be instances where you would have to tell somebody that their dream shouldn't be their dream and that would be really hard," she said.
Watch Carrie Underwood's 'Undo It' music video below: