Ford Fiesta while driving wheel pulls to one side, either right side or left side

I have an issue with ford-fiesta EXI 1.4 variant, When I drive for a while and if I keep steering straight car pulls to one side, either right side or left side.

Few things I have checked

1. Wheel alignment.
2. Tyre condition.
3. Power steering fluid leak.

All seems to be ok. Someone please give me suggestions

1. How to check if steering box is issue ?.
2. Is it safe to drive 700km ? with the above issue