choose to believe or not believe
Well, that 'choice' is based on comparing it to other possibilities, surely? Given my experience of people in general for the past 50 years. So, framed by my viewpoint/experience of what is likely and unlikely, which will never entirely coincide with anyone else's viewpoint/experience.
So we have one person's statement, and then imagine other possibilities, and then come to a conclusion about what is most likely.
So, my neighbour's string of cats that die might be because they are a multiple cat murderer. On the other hand, they might take in elderly and unwell cats to provide them with a comfortable life for their last weeks, under the supervision of a responsible vet.
In reality my neighbour has had the same cat for the several years I've lived next door, it seems healthy and terrorises my scaredy-cat of a dog!
But you only have word for it...