Ok so the court ordered drug tests are impossible to scam. Insist on a drug test. Nice easy. He will no doubt ask the same of you... Sure he can try and scam to get a negative result but it won't work.
What if he passes? No drugs? Can he see the kids then? OR what if you hire a PI and they find nothing? Look forget the PI. Why? well anything they find will likely be inadmissable as evidence anyways. So spend all that $$$ on a PI only to find out that it does nothing to help your cause. BUT more on that later...
Now is he trying to get custody? as in the kids live with him and never see you? or do you mean he wants to spend time with the kids? I'm guessing it is the second one. So if he is a drug addict, as in an $500 a day crack addict who would sell his own kids to get a hit, well different story. If he has a job, a car, stable accomodation but likes to have a joint every once in a while. But generally manages to keep apointments, pays his bills, well guess what.... Maybe just maybe he should also be allowed to spend time with the kids.
So you're despearate to find 'evidence' or a reason to stop dad from seeing his kids... The law states that it is a presumption that it is in the best interests of the child(ren) to have a meaningful relationship with both parents. That is the law. Now unless there is an established history of child abuse, neglect or something directly relating to the kids, you've got nothing. So, for example, dad found by the police passed out in the car with 30 empty beer bottles on the passenger seat. NOT A BIG DEAL. BUT, if the kids were also in the car... Then you've got something.
It seems your basic premise for not letting dad see his kids is because MAYBE... MAYBE. BUT with no evidence, just MAYBE he is selling drugs?
You really don't have much of a case. So based on what you've written here I'd strongly encourage you to stop wasting money on solicitors if it has come that far. Definately, don't waste $$ on a PI. And just let the bloke see his kids.