Okay.
For the moment don't try to surreptitiously 'test' him. Just interact as normal.
You might want to make a mental note of how he makes a cup of tea, let him lead wrt breakfast making, observe his decision-making. Make sure he takes his tablets/meds if on any.
A difficult, honest convo to be had with your Mum in the near future about how he is, does she have to do lots for him. Not in front of Dad, no-one likes being talked about.
Nb don't deny his story, it is his reality for the moment, just be reassuring (well they've gone now and taken their silly old smelly fish) and divert (oh LOOK was that a Bullfinch/woodpecker in the Apple tree) and distract (shall we pop to the paper shop to buy a paper)
Can you stay til Mum returns from herweekend away? (Might not be possible ofc)
Ring Doorbell for the front door. Nest camera system covering the back door, you can see comings and goings if set up correctly.
Make sure he drinks plenty, UTI can cause delirium in older people.
Bit of a ramble, stream of consciousness there, sorry.