How would a small nearby black hole be detected and confirmed as such? This article suggests that there might be a tennis ball-sized black hole orbiting the Sun somewhere beyond the orbit of Neptune. According to the article, observational anomalies of trans-Neptunian objects suggest that there is an object out there about 10 times the size of Earth (I assume it to mean about 10 Earth mass). Since searches have failed to find such an object, an alternative hypothesis claims that there may be a primordial miniature black hole out there. The article goes on to state that a black hole has not be found there because searches have been for planets and the methods used to find planets are different than those used to find black holes.

My question is: how would such an object (mini black hole) be detected, to either confirm or rule out its existence?
I would say that the only way this can be done is making ever more careful observations of the TNOs that are claimed to be affected by this putative black hole.

Ultimately such observations should narrow down the location of the object. Even then, confirming it's existence will be very difficult. It will have a small lensing/magnification effect on background stars, but it would have to get very close (in terms of angular separation) to a background star for this to be observable. It's unlikely that the position could be pinpointed accurately enough to predict when such a microlensing event would take place.

Perhaps a better bet would be to send a probe, like New Horizons, to its likely location, where the gravitational effects could be studied in situ.