How to check if a signal is in a clock domain

In reply to dave_59:

In reply to abasili:
You can use
always @(signal iff (!$isunknown(signal) )
to get rid of X transitions.

By the way, I’ve tried the
always @(signal iff (!$isunknown(signal) )
, but it does not seem to work properly and I get in the always block while transitioning from X to 0.