I think women are way smarter than men in terms of hiding this stuff.
If she really wants to find out, I think she’ll eventually find out one way or another. I think to a certain extent you need to have the kind of relationship with your SO where, to a degree, she doesn’t give a fuck what you’re doing, even if she doesn’t come out right and say it that way.
I am not saying that you won’t be in a world of shit when you get caught. Even if she doesn’t actually care about you anymore or your dick, she does care about what society thinks about her being cheated on.
One thing I have done in the past, that worked for me, is that I did not have all my money going to my main bank account. Some of it was deposited to a prepaid debit card, and the funds would gradually build up in there. I did not keep a record of that account or card, and the bank that issued it was obscure. Periodically, after a funds built up, I would withdraw cash from the card at an ATM, and pay in cash always.
This idea would not have worked with a SO who was reconciling our monthly budget in detail, because the funds to the prepaid debit card represented it an unexplained hole in the budget.