A quality distance learning program will offer a variety of features that others might not. Check to see if the school is accredited. Depending on the field you are going into, you might also need to attend a program with specialized accreditation. An example is mental health counseling. CACREP accreditation is a requirement for licensure as a mental health counselor in many states; therefore, CACREP accreditation in a mental health program would be an indicator that the program is of high quality.
Read up on the reputation of the school. The Internet offers a wealth of information on virtually everything, so chances are good that if the school has a bad reputation you might be able to find out by looking online. All schools are likely to have a few dissatisfied students -- sure to take that into account when completing your research.
Ask the school if it is willing to let you talk to some of the alums. Most schools keep track of their alumni, so this may not be as hard as it seems. Not everyone will be willing to take time to talk to you, but if the school offers a true quality distance learning program, you should be able to find a few alumni willing to talk to you and sing the praises of the program.
Has the school won any awards for the online doctorate degree program you want to attend? Have they won any awards for the school in general? Keep in mind that not having any awards does not mean the school is of poor quality. Awards are just something else to consider if they are present.