After freelancing for three years I've developed a sense for what sort of client to be on the lookout for when I read job listings. Basically I watch for professional clients who offer details and who are prepared to pay a reasonable amount for quality work. I've also picked up on things that tell me the client and I are likely not a good match: hints of micro-managing, rambling listings with no solid info on the work, or rife with claims of how busy or important they are, and of course my personal favorite, threats to prosecute me before I even start working for them.
There's no one yardstick to measure potential clients for anyone, but you can usually pick up on these red flags:
Crowd-funding payment: If you see crowd-funding mentioned anywhere in the listing for the project, make sure your part is fully funded first before you take the job. You want to get paid, not hope to get paid if everyone likes the idea over on Kickstarter.
Multi-staged auditions: Often clients will ask a potential hire to write a trial scene of a couple thousand words, for which they will pay only pennies. This is slightly annoying, but okay. Asking you to do that, plus a chapter if the trial scene is accepted, and then three chapters if the chapter is accepted -- you get the picture. This client can't commit (and I had a publisher do this to me for almost a year, once, so it's not bad behavior exclusive to WFH clients.)
Settling someone's score: An obviously angry client wants someone to write a story about something like, say, their horrible divorce. Before you get sucked into it, let me tell you what they probably really want: a very cheap therapist, or a co-conspirator in a revenge plot. Be neither.
Too much work, too little pay: No client wants to pay millions, but there are plenty who want to pay peanuts (aka a penny a word.) If you're a newbie ghost writer trying to get your foot in the door, you can go for these jobs, but stick to small projects until you've built up your resume enough to attract better clients. Two cents a word is standard, but three cents a word (while rare) is better.
Unreasonable expectations: Some clients seem to think writing takes no time at all, or that writers are like sweat shop workers. I saw a listing the other day for a client who wants five thousand words minimum turned in every day. For a penny a word? That's fifty bucks. You can make more than that slinging burgers.
Finally, trust your instincts. If something in the listing makes you uneasy, that's a red flag (even if you can't define why it unsettles you.)