Look at the "About Us" page for the online school you are considering. If the school is accredited by one of the national accreditation associations, the website typically features an official accreditation mark as well as a blurb about its accreditation.
Visit the U.S. Department of Education website (ope.ed.gov). This website gives you a full list of all colleges in the country that are accredited, including online colleges. This can help verify that the claim the college makes on its website is accurate.
Check the website for the association through which the college claims to be accredited. Six of these associations exist in the United States: New England Association of Schools and Colleges (neasc.org), North Central Association (ncacasi.org), Middle States Commission on Higher Education (msche.org), Southern Association of Colleges and Schools (sacs.org), Western Association of Schools and Colleges (wascweb.org) and Northwest Commission on Colleges and Universities (nwccu.org).