If your husband was having an affair, there would have been signs. We all can see them clearly once we find out about the affair. Before we figure it out, we are blind as bats!
1. First, and this is almost 100% of the time, your husband would have treated you like an annoyance. He may have become impatient with you, started finding fault, etc. They all do this. We all think that it is stress from work, but it isn't. Think back, did your husband turn on you, like you were the enemy?
2. Do you remember strange smells? I'll give you an example (go ahead and laugh!). When my husband was in the midst of his affair, from time to time he had the odor of a dog! And here's what was interesting, his clothes didn't smell like a dog, but he did! Now it has been years since the affair, so I have learned a lot about the OW. Her dog slept with her! So think about it, when he got in her bed, he wasn't wearing clothes.... Any strange odors? Think about it. Sometimes
they will take showers at the OW's home and they might smell like a different kind of soap. She may smoke.
3. This is a longshot, but I'm including it simply because your husband's OW sounds like she has plenty of nerve. Same here. I worked in a furniture store as an interior designer and OW came in posing as a customer. I didn't know who she was at the time. Any strange woman that showed up and just started chatting? Anyone that you run into a lot? Other women are obsessed with us and they want to know about us. In my case, she wanted me to look up her past sales in my computer and told me her name. When it didn't ring a bell with me, she became upset and quickly left the store (almost ran out the door). Then at a later date she came back and stood in the front of the store staring at me. Never said a word and then left. See if there is anyone that comes to mind in your situation.
4. I learned this from my detective. He told me that most of the time, all is well in affairs until the 8th month! That's when the other woman starts to put her foot down and wants to know where the relationship is going. Sometimes they don't get the answer that they want. Did someone enter your husband's life about 8 months ago? If so, there's a start! Look at his facebook page and she if anyone new appeared about 8 months ago.
5. Get on his computer if you can and look in the search bar or history for past searches. You can learn a lot by doing this. Go to his search bar and type the letters of the alphabet one at a time. A list will drop down showing what he has been researching. You might find that he has been investigating vacations, divorce attorneys, another woman's facebook page, etc. If you see things that a happily married man wouldn't necessarily look for, red flag!
6. Someone mentioned this, but I will too. On his phone, go to the Maps icon. Go to Timeline and a map will appear showing everywhere that he has gone that day. Pull up the calendar and you can go back and look at each day. You can go back for years! This is a real eye opener. If he has it turned off, turn it on! If the map shows that he keeps going to the same address quite often, that's where she lives.
7. A detective can mirror his hard drive and this is priceless. It shows everything that they have ever looked at, any picture that they have received, etc. All the erased things are there! It costs about $3000, though.
8. This is something that should have raised suspicion with me and maybe there is something similar that you have noticed in your marriage. Does he have a new interest? My husband suddenly loved Labrador retrievers. He posted photos on his FB page, he stopped to talk to dog owners if they had a lab, he couldn't get enough of those dogs! Guess who had a Labrador? Anything like that? Usually this is in the beginning of the affair when they are honeymooning.
9. If your husband is a man's man, and by that I mean he has lots of male friends, did he lose interest in those friendships? They do during the affair! I don't know why.
10. OW's cost money. Look at his bank statements.
Look for some of these. I'm sure that others here on SI have more. You will never get your answer from him if he is having an affair.