This is a couple warning signs to help you guys avoid people who are not going to pay you well and who might try to take advantage of you. These people lurk on deviantART to find new talent, assuming you are young and inexperienced and will naively accept crappy jobs. Don't fall for it!
1. They ask for speculative work.
Phrases like "see what you can do" and "find out if you're a good fit" are big red flags. Don't ever do free work for someone who is offering to hire you. Nine times out of ten hey will take the work and use it and never offer you a job. This is almost always a scam. Just say "I have examples of ____ in my gallery". (you should have examples up of every style you can do.) Don't even do SKETCHES for the guy until you've signed something or gotten some money.
2. They offer to pay you in advertising or give you the "honor of seeing your work in print".
It is an honor for THEM to be able to print your art, not the other way around. Seeing your work in print is nice, of course, but if it's not a good publication you're not going to want it on your resume anyway. Don't take payment in advertising either, unless you know the site has a high amount of traffic; most of the time it's not going to pay off.
3. They say "I found your work on deviantart and I thought you'd be perfect".
This is a warning sign, not necessarily a deal-breaker, but watch out for this phrase. To outsiders, dA tends to mean a big collection of naive artists who will work for little pay. Don't accept anything less than what you're worth, and if you're good enough to get offered a job you're good enough to get paid a fair wage.
4. They try to haggle with your hourly rate by claiming they don't pay any of their artists that much.
This is a completely BS tactic. Don't buy any baloney about how they don't pay their other artists that much; even if it's true, it doesn't matter. YOUR work costs as much as YOU price it, period. Don't let some random guy tell you how much your time is worth, especially since he's biased in the first place.
Once you've gotten your hourly rate, STICK WITH IT. Never go down, only go up, because it's ten times harder to go up again once you've gone down.
5. They don't want to sign a contract.
If they ever refuse to sign your contract, especially without reading it, or if they say "let's not bother with formalities", hit the road immediately. They are guaranteed to screw you over.
And yes, you should have a contract if you're doing commission work, even if you only use it for the big jobs.
6. If you're not using a contract, they don't want to put any money down.
It's standard for artists to ask for at least some money up front if no contract is involved, if not the whole sum. Don't buy their concerns that you will disappear with the cash. YOUR reputation is on the line, not your client's. If we disappeared with the money, do you think anyone would ever buy from us again? But clients disappear without paying us all the time, and we have little legal recourse against them if there was no contract. Get some money up front and don't send the full file until you've been fully paid.
7. They address you by your username.
I don't know why exactly, but this has always been a warning sign for me. You should have your real name up on your site if you're doing business. If they don't even bother looking for it, there's a good chance that they're going to be a crappy customer.
8. They want to pay you by the week or the month instead of the hour.
This is just code for "we're going to unload a crapload of work onto you and pay you peanuts for it, and then fire you." Your pay should reflect the time you put in, not a flat rate.
9. They won't talk to you on the phone or give out their real name.
Granted, there are a couple legit reasons why some people can't talk on the phone (language barrier, for example) but if they "just don't want to give out our number to someone online", they're going to scam you. If they don't trust you enough to give out their phone number, why should you trust them? Not giving out a phone number means they can disappear entirely. Never operate with just an email address.
And if they don't give you their real name and company info, they're a faker. Run like the wind.