How do you determine the half-life of a radioactive parent isotope based on a decay curve?

If you have the curve on a graph, then it should be easy.

-- Pick any level on the y-axis, like a level that's marked with a
       horizontal grid line.
   Find the point on the graph where it crosses that level.
   From that point, look down on the x-axis and find the time at that point.

-- On the y-axis, find the level that's 1/2 of the first one.
   Find the point on the graph where it crosses this new level.
   From this new point, look down on the x-axis and find the time.
  
-- The time interval between the two points is the half-life of that isotope.