IF (X .NE. X) THEN	! this will be true if X is NaN 

 program test_nan

        implicit none
        real*4 x
            x = -100.0
            x = sqrt(x)

        if (x.eq.x) then
        print*,'it is not a NaN'
        endif

        if(x.ne.x) then
        print*,'it is a Nan'
        endif

        end program test_nan