Pressure test the cooling system as mentioned above. That’s where I’d start. White smoke out the tailpipe may be a headgasket problem but can be caused by other things too, for example the intake manifold gasket. And coolant might not even be involved. One test you can do is warm the engine up, then drive up a freeway on ramp accelerating rapidly, with a friend following behind watching your tailpipe. If the white smoke really starts pouring out the tailpipe, whatever the problem is, it has to be addressed asap, b/c this can damage the cat.