In EGA IV, 17.2.6 the following characterization of monomorphisms is given:

> Let $f : X \to Y$ be a morphism locally of finite type. Then the following conditions are 
equivalent:
 
> a) $f$ is a monomorphism.
 
> b) $f$ is radicial and formally unramified.
 
> c) For every $y \in Y$, the fiber $f^{-1}(y)$ is either empty or isomorphic to $\text{Spec}(k(y))$.
 
Also note that (due to the adjunction) a morphism between affine schemes is a monomorphism if and only if the associated homomorphism of rings is an epimorphism (in the category of rings) and the latter ones can be characterized in many ways. See, for example,[this][1] Bourbaki seminar and [this][2] MO discussion.

 [1]: http://www.numdam.org/numdam-bin/feuilleter?id=SAC_1967-1968__2_
  [2]: http://mathoverflow.net/questions/109/what-do-epimorphisms-of-commutative-rings-look-like