Have you been offered a really good opportunity, but are not sure whether it's honest or not? There are some simple ways to tell the difference between a legitimate offer and a scam.
Take a careful look at the offer. Ask for a copy of the contract if one is available.
Read through the contract, making notes of things that don't seem quite right to you. Highlight parts of it for future reference. Don't worry if you can't put your finger on the reason why something isn't right, just highlight what makes you suspicious.
Do some research about the company. If it's a new company, be suspicious. Go to it's website. If there are ads and things on the page this suggests the company isn't making enough money to buy a domain name and support it without ads. Be suspicious of ad-riddled pages or a sloppily-designed site.
Ask if they have any brochures, etc, for the company. If they do, this is a good sign.
Ask people you know in the industry what they think of the offer. They might know if it sounds legit or not.
Go back to the contract you reviewed and do some research about the specific sections that made you nervous. Talk to the company about these sections. They should be business-like, and professional. If you feel like they are trying to sweet-talk you be extremely wary.
If you feel it is necessary, talk to a contract lawyer or a friend experienced in this area.
Check out the company with the Better Business Bureau (or the equivalent consumer protection agency in your country).
If it sounds to good to be true, it is.
Be wary of large amounts of money. These can be the sign of a scam.
Don't leave a perfectly good job for a possible scam. Make sure you have checked out all the details.
If the offer is for employment or they are going to pay you for a service, make sure they have a contract. If they don't, refuse the offer unless they can create a legit contract.
Go with your instinct. If something doesn't feel right, don't dismiss that to quickly. You don't want to be stuck in a terrible situation.
If you are really worried, check out the backgrounds of the founder and employees of the company, and learn about the company's history. If you discover any criminal records related to the high-up people in the company, run away fast.
Some scams will try to sweet-talk you. Don't be fooled. If they do anything out of the ordinary for an employer offering a job (buy you presents, offer bonuses, etc) be extremely leery.
Never get involved with a business that wants money from you, legitimate businesses will never require money from you.