How can I verify if a self-defense school in Thailand is approved for an Ed visa?

The school will know if they are qualified, and registered with the ministry of education, to provide ED visas. If the school doesn't know then they are not.

I can't advise you on how to tell if a school is government approved, but I can advise on how to tell if a school is legit:

1) They have an attendance policy and strictly enforce it

2) There is no additional fee for "visa conversion." The cost for ANY visa in Thailand is 2000 baht paid directly to immigration, and the cost for ANY extension in Thailand is 1900 baht paid directly to immigration. If they are charging you any fees on top of this just for your visa, they are either taking advantage of you or paying a bribe to immigration to get your visa.

3) They take copies of your passport and send it to the Ministry of Education to get approval for your visa. Once they get approval they work up a LARGE packet of documents (100-200 pages) and give it to you so that you can go to immigration and apply for your extension. If they send you by yourself with this packet then they are legit. If they want to send a staff member with you they still might be legit, but if they charged you "visa conversion fee" and also want to send a staff member with you, there may be bribes involved.

Also if there is no requirement for you to go to immigration at all, run away! That is definitely NOT legit.

4) Your visa will be for 90 days and you will have to apply for an extension every 90 days. If they say they can get you a 1 year visa, then it's not legit for a language school. Only universities can do 1-year education extensions.