A lot of them are fake checks with actual bank information for the organization, a lot will be local to you to help persuade you to believe... A lot will also be actual business checks.
You really need to lookup the actual companies address and telephone number, call in and ask to talk with their accounting department and explain the check you just received.
If the check is legit, the accounting department will know, they are in charge of the money afterall...
If you get taken by one of these scams, you need to file a report with your local police department and possibly with your local fbi's cyber security department. Remember that crimes that are committed over state lines are federal crimes, especially when we're dealing with large companies.