Question

How can we determine the equivalence point ( or end point ) in the curves in titration ( strong acid/weak base and weak acid/strong base )

Answer 1

Do you mean how to calculate the pH at the equivalence point or how they look? Here is how they look; i.e., its' the mid-point of the almost vertical part of the curve.
https://www.google.com/search?q=titration+curves+images&client=firefox-a&hs=PS0&rls=org.mozilla:en-US:official&channel=sb&tbm=isch&tbo=u&source=univ&sa=X&ei=CIMSVPy7I4S1sQSFvYHACQ&ved=0CB8QsAQ&biw=1006&bih=602&dpr=1.25

If how you calculate it you "hydrolyze" the acid or base salt; e.g., for weak acid/strong base such as HAc where HAc is acetic acid, you have
HAc + NaOH --> NaAc + H2O. So at the equivalence point you have ONLY NaAc and H2O so the pH is determined by the hydrolysis of the salt, in this case it is
.........Ac^- + HOH ==> HAc + OH^-
I........C...............0......0
C........-x..............x......x
E........C-x............x.......x
Then Kb for Ac^- = (Kw/Ka for HAc) = (x)(x)/(C-x). You will know the concn of Ac^- and Kw and Ka for HAc and you calculate x = OH^- and convert to pH. That gives you the pH at the equivalence point.