Find out where the online college is physically located. If the only information you can find is a P.O. Box or an address in a foreign country, it is likely a scam. A legitimate online college will at least have physical office buildings within the country.
Check out the school's accreditation. It should be backed by an accreditation board with a good reputation. You should research the accreditation board as well, making sure that it is associated with traditional colleges, not just online schools.
Be wary of a school that promises you a degree based on your life experience. While many colleges will award you some credits for your on-the-job knowledge or exempt you from certain courses, you cannot trade in 10 years on the job for a bachelor's degree. You must do the work, take the classes and pass the tests. Any online school that promises otherwise is a scam and your degree will not be real.
Pay attention to how long they say it will take you to earn a degree. An accelerated program may allow you to complete a bachelor's degree in three years, but if they promise you a bachelor's degree in a year or less, move on to another option. On campus at a traditional college, a bachelor's degree generally takes four years to complete. Remember that if it sounds too good to be true, it probably is.
Read over the tuition costs carefully before sending any online school money. If the school promises you a bachelor's or master's degree for one flat rate, it is probably a scam. Traditionally, colleges charge you per course or per credit hour. You may see a cost per semester and that is usually OK, as long as it specifies a number of courses or credits, such as "Each semester costs $4,000 for 15 credit hours."
Do an Internet search with the name of the online college and the word "scam" in quotation marks. You will easily find out if anyone is complaining about the school. Check with the Better Business Bureau (BBB) as well to see if there are any complaints. If you scour the Internet and check with the BBB and don't find any complaints and it meets all your other criteria, it is a pretty safe bet that the school is legitimate and you can pursue a degree.