Only lawyers really know how to vet lawyers for competency, I've found. More people use the quality of the jingle as a guide than any useful way to discern competence, partly because people broadly fail to understand what a lawyer is supposed to do (and this differs depending on context, civil proceedings and criminal proceedings and administrative proceedings concern sometimes vastly disparate interests and in turn, responsibilities and expected outcomes).

The best way, generally, is to survey other lawyers in the same field and jurisdiction and cross-reference to see if a firm or solo practitioner gets more recognition from their peers independently, btw. It's how lawyers find representation in fields they have no familiarity with. The jingles at best are entirely irrelevant and may even be an indicator that the firm relies on volume more than it really should.

edit: should mention that this generally applies for criminal or administrative cases. The civil arena is far too big and varied for me to suggest a heuristic that can be generalized since it concerns so much work that is wildly different and also, comparatively few court appearances (although I'd likely use the same method, but starting with my classmates, if I need to hire someone for a civil matter, if I have a classmate who practices in the field and in my jurisdiction).