For us, I think when we have those kinds of lapses, if there's other 'symptoms' going on at the same time, we'll be more likely to attribute it to DID stuff.
Eg. we go on bike rides every morning. We've had our bike for like 18 or 19 months now, riding it every day. One morning I went out on the usual ride, but I could feel a sort of impending anxiety/panic attack forming which usually means Arin is close to front, or possibly triggered. And at the same time, I forgot how to operate the bike's gear shifter and brakes.

I also noticed our legs felt really weak and we were really shakey on the bike, but at least we didn't fall off or anything. I actually had to stop and stare at the shifter on the handlebars to figure out how it worked, and for the whole ride we had trouble. The next day, no Arin and no troubles remembering how the bike worked.

A more significant example would be our previous host was really talented with computers. Programmer, systems designer, all that stuff. She disappeared late last year, and V1 and me ended up taking over, but neither of us are as good at computers. We're just like slowly figuring stuff out that she could do without even thinking. So, not like a brief moment of forgetting something, but a skill that's important to our job has basically gone missing. It's not like it's entirely forgotten and gone forever, there's another part who seems to have the computer talent, but she doesn't front that much.
V2