TCL command to check for clock assignment

This is not quite what you are asking, but I typically use the .sdc syntax:

set ports 
if { == 1} {
	set clkin_50MHz_period 20
	set name clkin_50MHz
	puts "  ==> S4GXDK: constrain clock $name"
	create_clock                    
		-period $clkin_50MHz_period 
		-name $name 
	set_clock_groups -exclusive -group $name
}