put some paint or tipex on the crank pulley (this been the one at the bottom that connects to the crank), line up the cam with the timing mark on the cam and the rocker and finally line up the timing mark on the fly wheel as well, well they all should line up and when they do you shouldn't have any timing problems.
i did this with my engine rebuild and we took the cams out of the head and put them back in and made sure the marks were all in the same place and it was sound