What are the ways to determine the processes that have been swapped by the kernel?

One easy way is to look at the output of ps aux for processes with a 0 (zero) Resident Set Size (RSS) or a state of W (which means the same thing).

There are probably other ways, but they all involve using tools or utilities that look in /proc for processes with a RSS=0.

If the goal is only to detect processes that are completely swapped out...