This bothered me to because it's a common cop out. I bought a book on lying and it had a chapter about not remembering.
It said you forget things for two reasons.
1-Because it was a long time ago. A long time ago is relative and differs for each person. They pointed out if the person can remember details from the time you are asking about but just not the details you want, odds are they are lying. But you also have to consider the second reason.
2-Because the information wasn't important to you. Again, this is relative so not something you can definitively pin down. If you ask me what color was the car opposite me at the light when I stopped to get milk this am, I wouldn't remember because it didn't matter to me. The comment above that someone made about knowing the date they made the biggest decision of their marriage doesn't work because of this second reason. Cheaters aren't honest with themselves about what they are really doing. It almost certain he didn't feel like he was making the biggest decision of his marriage that day so that reason for remembering won't work. What he might remember is their first time together. Just reframing the question sometimes works.
You have to take both reasons and carefully mix them with your gut feelings. Sometimes it's obvious, like this time with my wh.
I asked about what he did at a strip club. He claimed he was too drunk and it was too long ago (approx 12years) to remember. I asked for every detail he could recall.
He started spouting off with very detailed info regarding what the bus driver did in the club. I'll just say it was detailed info.
Anyway, he was proud of himself and said in a snarky manner that I wanted Allthe details. At first I was ticked off because who cares about the bus driver but then I realized he gave himself away. It wasn't too long ago and he wasn't too drunk to remember very detailed information. So I called him on it and after several more arguments, after he realized I wasn't going to drop it, he told me what he had done at the strip club.
It's possible he didn't tell me everything or he made some things up. But he knew I knew he was lying when he said he couldn't remember at all. He had to provide some details.
Sometimes clinging to the lie is more important than hiding the details.
So consider those two things when you get the I don't remember answer. Keep in mind they have to be important for the cheater to note, and not what's important to you. Find a way to ask the question from all different angles. If pieces don't fit in from other versions, then push for more clarification.
Ask leading questions. Let them do the talking. Make note, of everything they say. It's ok to come back after a discussion and say, hey this doesn't make sense to me, please clarify it again.