It definitely helps if your partner simply does not think you would be that nasty, as you come across as a decent, honest person. Even though I could see some things my husband was doing - sending text messages to a woman - I was willing to believe his story that he was talking to her about his mother's and her father's illness, as they were going through the same thing. I thought he might end up having an emotional affair, but I didn't think he'd been going to hourly sex hotels with her for months and months, as I honestly couldn't imagine him doing anything so low down and disgusting or using his dead mother as an excuse for it. I did complain, but he would always bring up his dead mother to make me feel I was the one being unreasonable.
It helps if you have always had irregular or very long working hours, with long business trips so your spouse has got used to you coming home at all hours or not at all. Just add some comments about how you have to do someone else's job as they are seriously ill, and (again) your spouse will feel bad about saying anything.
Complete change of clothes, look, weight loss, manly moisturiser, new beard? Your mother's death has made you think about mortality so you want to take care of yourself.
Shutting yourself in your room with the computer to write long emails to your mistress about your bitch of a wife, never once mentioning your dead mother but instead writing pornographic novels about your mistress's "pussy"? Tell your wife that you have realised you never get any time to yourself (i.e. make her feel like a nag if she complains) and that you need time to grieve over your dead mother. Tell her (indirectly, of course! Don't want it to be too obvious) that she doesn't understand, as her mother is still alive. Then if she repeats that back more directly, tell her how hurtful she is for saying that. Generally play lots of mind games so that she doesn't know who it is being nasty any more.