Buy from a European supplier - testing is mandatory and all e-cigarette products must be registered with the national healthcare regulator prior to sale. It's not an ironclad guarantee of safety, but we have an effective regulatory regime.
The only good way is somehow lucking out and finding an independent researcher that tests them without financial incentive from the company. Otherwise typically the findings will always be on favor because companies request preliminary data and pull funding to continue research if it looks bad so it doesn't get published
