I think it would be better to say 'You need to be a good friend to your gut', rather than 'Your gut is always right'
So, imagine if some stranger in the street came up to you, drunk, and said 'You're husband's sleeping with my best mate', you would dismiss this because you're not her friend, you don't respect her, and, frankly, you just think she's being a bit stupid and causing trouble for no reason. Lots of people have this response to their gut feelings. They basically are happy to disregard their own emotional self as some kind of drunken street prat.
Now imagine if your long time closest friend came to you, sat you down quietly, got you a drink, and told you she had something hard to tell you. 'I'm so sorry, but James has been having an affair with Sue from the office. I told him that if he didn't tell you by today, I would, because I can't bear to see you being betrayed like this.' Are you going to disregard what she says out of hand, like you did with street prat? No. You're going to take her much more seriously, and tell James what she's said, to guage his response.
That's how to deal with your gut feelings and suspicions. They might not always be right. They don't just make crap up for nothing. It might be that he's having an affair, or it might be that you have some insecurities to look into, but either way, listen to what your gut tells you.