The United States Department of Education, through its Office of Postsecondary Education, monitors the "accreditation" of schools. Accreditation is handled both by geographically regional organizations---not always relevant for online schools---and by organizations specific to that particular field of study. Rather than relying on the school's brochures, you can use the Department of Education's website to check on the accreditation status of an online college.
There are two types of online colleges. The first type are those schools that are only online, with only a limited or perfunctory physical presence. Most of these are "for-profit," which means their primary goal is making money. On the other hand, some schools are not for profit. The Western Governors University, for example, is an online-only school founded in 1996 by an association of the governors of western states.
Some traditional "brick and mortar" schools also offer online degrees, which have the advantage of carrying with them the legitimacy of an established institution---you can generally assume that an existing, traditional university is reputable. In some cases, courses are delivered entirely through distance learning methods. In others, most lectures and tests are conducted online, but you may be expected to spend a few days on campus for a seminar or activity that can't be carried out well online.
Finding a reputable online college begins with accreditation, but you should also look to see what other people are saying about the school. School rankings, such as those published by "US News & World Report," can be a helpful tool in figuring out which schools are the most well-respected. Many ratings will also show what the school's strong suits are. A school may have a very good MBA program, but its pre-law course offerings may only be average.