We can observe these black holes, if they existed, via their lensing effect on light passing near it

If you watch the night sky very carefully (with telescopes of course) and train it on a faraway galaxy, you will be able to observe the mass of anything that passes between you and that galaxy. Any massive object would bend the light eeeever so slightly, and you would see the galaxy wiggle in the background

Extremely small black holes would need to exist in such density that they’d need to permeate everywhere. Excuse for a moment that they’d need to be passing through our solar system regularly (this is fine, but we’d detect them no problem), their effects on viewing distant objects in the cosmos would be unmistakable

Also, we can rule a HUGE range of masses for black holes because anything less massive than ~200 million metric tons would’ve evaporated by now. Black holes of this mass would be the trickiest to spot, but fortunately, Hawking has proven they literally can’t exist if they formed in the big bang

edit: another problem with small black holes is that they’d have charge and they’d rotate, both of which make them MUCH “clumpier”. We’d end up with swathes of intermediate and supermassive black holes (MANY more than what we currently see), and these would be trivial to observe