If they are an actual carrier you will see a classification of 4561-000:General Freight Trucking on their certificate. If they do not have this classification then you should suspect that they are not an actual carrier because all their staff will have to fall under this classification unless they get approved for a second classification which is tough.
The problem is what the question of what they do report. If you have a carrier that is 100% Owner Operators and the O/O are legitimate those are truly vendors and the Carrier would have to prove that they are legitimate and no WSIB premium is due. If they are paying via Drivers Inc, and report and pay WSIB on the wages paid to Drivers Inc., they are in compliance. The WSIB doesn't care about CRA issues.
This is why Driver Inc is like cancer in our industry. We don't know who is utilizing it, we can't weed it out and as time goes on, those individuals will get caught. Some of those caught will come around and pay legally, others will not and fight the system to the death. It is a long tedious process and only more enforcement by the CRA to ask Driver Inc. companies for their books and audit the carriers utilizing them.
Call your MP and ask your MP to get the CRA working on Driver Inc. issues.
You will not know for sure . One example is ; A carrier can have 20 drivers and 50% are employees and other half are Driver Inc . They will send you a WSIB clearance and it’s good and updated , but it only covers the employees. How do you know what driver is pulling your load ? You don’t . Also Jim is correct on some carriers paying WSIB on Driver Inc . They just don’t pay or take any other source deduction. But they cover WSIB because they don’t mess around like CRA does .