When I see a car that seems to be putting out a visible and/or smellable amount of smoke/soot/smog, are these the worst polluters? I guess what I'm asking is i

When I see a car that seems to be putting out a visible and/or smellable amount of smoke/soot/smog, are these the worst polluters? I guess what I'm asking is if visible inspection is a reliable way of detecting these vehicles?