It can be really hard because you have to depend on people to be honest, there is no way to prove if a dog has seizures or not. There are some foreign databases that can be really useful for Tervs and Groens but sadly, very few Mal or Laeken people seem to participate in them. Which might give the idea that epilepsy is not a concern in those varieties and I really wish that were the case. It would make it easier to improve if two of the four varieties didn't have an epilepsy issue. I think it has to do with so many Mals being bred by people who are in the business of breeding and selling working dogs. It looks bad for business to be too upfront about any potential issues. Also, breeders who keep their dogs kenneled may never see seizuring if it did happen.
It wasn't all that long ago that Groen and Terv people weren't all that forthcoming about seizuring either. I remember when I first got involved, the Terv people used to claim it was an issue in Black Dogs but not Tervs. And the Black Dog people would say it was "a Terv problem". I'm glad the hairy crowd has become more open but there still isn't total honesty there either. Looking at the databases, there's very few American dogs listed with epilepsy compared to European dogs. Of course there's fewer dogs here but I personally know quite a lot of dogs who seizure or produced it that aren't listed.
Here's some of the databases:
http://www.belgian-epilepsy.dk/liste.htm
This is probably the biggest database but just a handful of Mals with epilepsy listed, some working and some show. You can search for Mals and pull down the second choice bar for "with epilepsy".
http://baza.belgi.pl/
So my answer is, I wish I had an easy answer for you. Your best bet is to try to find the most honest, upfront and knowledgeable people involved with Mals and see what they are willing to tell you. At least with your own dogs to start with. You can ask on different email lists, FB groups or forums if anyone is willing to talk privately with you about pedigree research regarding epilepsy in working line Mals. Hopefully one day, we will have a genetic marker for epilepsy in all of the Belgians and will be able to make truly accurate breeding choices regarding epilepsy.