You can never know anything for sure. This is why science, medicine and drug approvals are all about probabilities and calculated risks. If everyone suddenly dropped dead 10 years after taking a drug that was approved in 2015 there is no way we could know that right now. But we know it's very, very unlikely, so we accept that risk.

With vaccines we know from decades of experience that severe side effects tend to occur very shortly (days or maybe weeks) after the injection. So this is why vaccine trials observe participants for weeks or months, not years, before concluding that the vaccine is safe. Of course this doesn't guarantee that the Covid vaccine don't happen to be an exception. But if that was the approach we took then we would never be able to approve anything, and never get the benefits we know for sure it brings.