> I don't even really know how to hire a lawyer or accountant, or how to assess whether the person I pick is minimally competent to give me good advice
Reputation is one proxy you can use for trying to gauge competency - ask friends, family, or coworkers if they have someone they're happy with.
Ultimately, there's a few big things they need to do, and you can try to evaluate those things instead or as well:
1) Clearly explain things you need to know. This might be making you aware of paperwork you need to file, tradeoffs between choices, rationale behind why they're suggesting you do something, or what the tradeoff is so you can make your own decisions.
Some (many? most?) lawyers are willing to do a free initial consultation, so you can get a feel for this in person, if you don't have a recommendation or don't want to trust only reputation. Basically: Can they communicate effectively?
2) Keep you out of trouble. There's a limit to what a lawyer or accountant can do of course, but a pattern of surprise financial difficulties, or getting into frequent contract disputes or lawsuits are a bad sign. The IRS frequently correcting an accountant's filings would be another.
3) Stay on top of things. Is the accountant filing taxes late resulting in IRS fines? Paperwork not ready when they said it'd be? Asking for the same information several times because they keep misfiling it? Going incommunicado for months when you have questions?
It seems simpler in practice: It came up in conversation that my friend had gotten his will done. I mentioned I needed to get my will done. He liked his lawyer, and was able to explain some about what he liked about her approach and give me her name. So I scheduled a consult with her on her website after trading some initial questions over email, showed up as scheduled, and then traded more questions at her office. I didn't know all the right questions to ask - mine were mostly of the form "uhh, so what do I do?" - but she was able to volunteer explanations about state inheritance law that put her questions for me into context. And then she had recommendations for things to do and why, as well as things she'd recommend not bothering with and why (and the contexts under which that could potentially change in the future), without being pushy.
Based on my answers, she drafted a will for me, and I was able to come back and sign it a month later when I scheduled it. I think I paid half after the initial signing but before she drafted the will, and the other half after the signing? Now I can recommend her to my friends, or ask her if she has any recommendations for an accountant, or even other lawyers for specialized needs.
Could I have done more to vet her? Sure. But I did enough for my tastes. Can't let the perfect be the enemy of the good enough and all that.