I don't know about other industries, but in oil & gas this issue is prolonged since I am not sure when and I think it has become common. If you work in the oil & gas, the chances to receive such scam is high, it will be even higher if you an avid job seeker. It happened to me twice.
The first scam I got in Nov 2009, from a recruiter called Greatjobsdiary based in Scotland. I was applying the job through a forwarded email in my Yahoo group. A month later, they emailed me that they are interested to interviewing me through Skype due to great distance between us and in order to save cost, convenience and accessibility. I remembered their so called recruitment expert, Mr. frank Otieno, a Scottish guy. I barely comprehend anything that came out from his mouth during the interview due to his strong Scottish dialect. 2 days after the online interview, I received another email from them saying that their Australia clients are interested in having face to face interview with me in Brisbane. I started to feel doubtful when I read I have to pay upfront for air ticket and any related logistic matters which cost around USD1,839 which is of course they said refundable during the interview. I later confirmed it was definitely a scam when I emailed one of the client in Australia and the HR confirmed that his company has never engaged Greatjobsdiary for recruiting.
One of the sign that is definitely a scam. Candidates will never have to pay some amounts of money upfront prior to interview or relocation (in case you already secured the job). I had interviewed with Shell, they paid everything from air ticket to accommodation. Only the ground transportation such as taxi need to be used own money and to be claim later. I have also recently received an offer from automation company based in middle east. The company will pay around equivalent to RM20K for my relocation (Visa, passport, goods delivery etc) which of course excludes the air ticket for business class.
The second one, also happened in 2009, a company called elite-petrochemical limited based in Nigeria. They claimed they are supplying manpower for project code name: Escravos Project where Chevron is the biggest share holder. The trick is, I filled in the application form and answered some questions in the given form. Around a month later they come back to me with an offer that looks too good to be true. Why? With no interview either face to face or phone and no technical assessment and all, they willing to offer me as much as USD8,000 per month. I find that so ridiculous and backed off. However, I did replied them. I knew that the then group company I worked in was involved with Escravos, so I threw them a question "As far as my knowledge, the project has successfully delivered earlier this year, if I were to accept your offer what would exactly my scope of works there?" They never replied back!
Besides above mentioned signs, you can do some detective works by yourself if you encountered this situation or received offer that so doubtful. Well, google it. Try to find as much as possible about the company. Make a few calls if necessary to find out whether the company is legitimate. Some stupid scammers used commercial email in dealing with the victims. You must be aware of this. Sometimes you can see, for example: Shell Careers, but actually its email email@example.com. You know what I mean. And the most common way is to ask around. Ask your friend in the industry about the offer, ask the seniors. They probably know better.
Do not fall in their trap, this is something that see-able and avoidable and predictable.