Can I find out whether my ex does have a medical marijuana card?

50/50 custody. Ex a habitual pot smoker who claims he got a medical marijuana card for back pain after a car accident so that now I won't be able to badger him about getting high when he has the kids. How can I find out if he is just bluffing? And if he isn't, do I have any options to keep him from smoking around the kids? (This is in California)