WDs in binary with pulsars is actually how we've found many of the Helium WDs known to exist, since isolated Helium WDs are difficult to detect. Your question is a good one, though, and gets to the difficulties in detecting really any isolated end-products of stellar evolution unless they're interacting with something else. Stellar mass black holes by themselves are obviously invisible, and we can only infer their existence from micro-lensing events. Can we detect WD+NS systems that do not include pulsars? Probably only if it's an eclipsing binary. Assuming such systems are abundant (and there's good reason to believe this), a telescope like Kepler should be able to detect them, if only it was looking at WDs.

Cody.

Have the existence of stellar mass black holes been confirmed by detection of microlensing events? I find a program proposal intendend to do so - http://adsabs.harvard.edu/abs/2011noao.prop..460S

and a report of as yet unconfirmed potential observations, http://dx.doi.org/10.1086/342225

James,

There have been a number of microlensing detections, but their candidacies as black holes are somewhat ambiguous.