At AgentQuery.com I noticed a few agents had items in their profiles saying they were most interested in sticking by writers over the long haul - building careers, etc. I thought all agents wanted that? Do there, in fact, exist agents who prefer only to help first-timers get their start and then let them move on to longer-term agents - a "starter marriage" of sorts?
And is that one of the questions new writers should ask... not just what you've sold, but how long your clients have been with you, "relationship" philosophy, etc.?
I've never felt the need to say "I want you for the long haul" in my agency listing cause I thought it was akin to "I want you to produce good work"...obvious. I can't imagine a "starter marriage agent" by choice. For one, it's cost ineffective. Most novelists don't get out of the red with me till their second book, and even then there's usually a paperback deal or some sort of other deal that has helped. (The novelists themselves make money; remember I only see a portion of the proceeds AND I've invested more than one and less than one million hours in you).
As to what you should ask a prospective agent, you've got to remember that you cannot, absolutely cannot determine if something is going to work without fail. You take a risk when you sign with someone, as we take a risk signing you.
The questions ahead of time should first focus on making sure the agent runs their business responsibly: accounting practices, business structure, codes of ethics etc.
Second, you should make sure the agent's business practices mesh with what you want: does she invite you to participate in strategy, share rejection letters, stay in close contact etc, or is she like Miss Snark-cool as a cucumber to all advice and input and much more "I'll let you know when I have something for you to chew on".
After those questions, you just have to sacrifice a goat, say three Hail Marys, chant to the east, return all your library books on time with dollar bills tucked in them as a mitzvah, and hope for the best.