Look at http://teflcoursereview.com
which has former students rate their course providers.
My personal opinion is that online courses are ideal for entry level TEFL certificates as they provide a way into the industry for newcomers. A Diploma, however, is a higher qualification and whilst a lot of it can be done online, there should be considerable teaching practice included. So, this being the case, the only way an online course can do this is by having lessons videoed and then sent for assessment.
Do the diploma courses you are looking at have teaching practice assessment? They should do.