Here are the things I've seen work well from both sides of the hiring process, and from talking with a bunch of people about career choice.
If you are thinking of going to school for a couple of years to qualify you for a job, apply for that job today to find out whether you actually have a skill gap. If you don't know exactly what job it is you want, figure out what the fastest way to find this out is - it probably isn't going to school for years.
If you don't get job interviews (or even if you do), ask people for informational interviews. Ask your friends or acquaintances for introductions to the most relevant people you can find. Ask those people for a 30 minute phone call or coffee meeting. Ask them all the questions you're actually curious about. "Based on my experience, does it look like I'd be qualified? If not, what would I have to do to be a good candidate?" Ask for feedback on your resumé. If you're not sure you want to work in this field, make a list of all the things that would persuade you one way or the other if you found out about them, and ask those questions. At the end of the interview, ask, "whom else should I talk to?" If they're doing X and you want to do related thing Y, ask whom they know who does or knows about Y. Finally, if you think you want a job in their field, ask, "Who do you know who's hiring?"
If you're trying to decide whether a company/job is a good fit for you, that's what the job application process is for. Make it clear what you're looking for in your cover letter. If not having X is a dealbreaker for you, mention that you're looking for X. This way you waste less of your and their time, and if it is the perfect job for you, they have some evidence that you're an unusually good fit. Go into the interview with a list of all your actual, genuine reservations about the job; that's what you should ask in the "any questions for me?" part.
To illustrate, here's the cover letter I sent to GiveWell:
I want to do the best thing to help the world, but need to be able to judge competing claims to even know which interventions do the most good, and which skills I need to contribute to them. GiveWell and GiveWell Labs are doing that kind of work - I want to get practice with this.
I don't know if the best thing is for me to work for GiveWell, volunteer, or something else, but I'd like to start the conversation.
Personally Working With GiveWell
I currently manage an analytics team at Fannie Mae, I've built predictive models, and done empirical research to identify credit and collateral value risk. I've presented work to senior management a few times, so I have experience communicating quantitative results in clear plain language too.
I haven't done much work on closely related problems to the ones GiveWell works on, but here's my first public attempt to get a handle on comparing the magnitude of existential risk with other problems one might want to solve.
My resume is attached.
DC EA Involvement
On a related note, Matt Dahlhausen, a member of the DC Effective Altruists group and an organizer of the Effective Altruism group at the University of Maryland, has expressed interest in a group project doing research for GiveWell.
I don't know if GiveWell has gone in for this sort of thing before, but it seems like it has two possible advantages. Free labor (disadvantage: possibly poor coordination and QC), but maybe more importantly, building connections with effective altruists in DC. I think a strong active group of DC effective altruists might be able to do a tremendous amount of good, and having a real project to work on would help with group cohesion. Please let me know if you're interested in pursuing this.
I'm in the San Francisco area over the next few days (leaving on the 16th)--I'd love to meet if this is convenient for you.