Adding to that....
Think about what a push-pull amp does. When one side pulls low, the transformer pushed the other side up. Depending on how close the "on" side pulls the drain down, it will push the other side up.
The issue, just like with tube amps, is the Q in the load. If the load stores more energy than it absorbs, the drain voltage can actually swing negative. When it swings negative, it also allows the other side to go higher than double.
This is why U-tube videos showing 99:1 SWR and other things from marketing departments ruffle feathers of anyone who has looked at amplifiers in working conditions with modest or high Q loads. If we dumped into a short without enough Q for ringing, and limited time or current to not overheat the junction, an FET could take a terrible mismatch.
So we watch heat, and we watch or limit excessive voltage, and SWR doesn't matter.
For example, we ran very tiny FET's in meter magnet "chargers" that could add or subtract from magnetic flux to calibrate meter movements. We ran dozens of times the rated current, because we hammered the FET into and out of conduction, and the backpulse was dampened by a diode. This was nearly infinite SWR and many times rated drain current, and we used dozens of machines on the assembly line without FET failures. This was in the early 1980's with garden variety cheap plastic audio and low frequency JFET's. They weren't even power FET's.
You can measure at the drain with a normal scope provided the probe is suitable. Normal peak voltage should be a bit less than twice supply voltage.
Look here to see what happens if you have the wrong load:http://www.w8ji.com/demonstation.htm