how do you find top dead center, to see if timing has jumped.

What is your question?

I came in from worked,Changed clothes,got back in my 1998 G.M.C. safari van. It would not start. I let it set til next morning. Still wold not start. I pulled the cowling out of the center,changed cap and rotor. still don't start. I am getting fire from #1,but still will not start. How do I find "TOP DEAD CENTER".