One of the most know frameworks for validation  is
http://jakarta.apache.org/commons/validator/

Look at the sample at below address:
wiki.apache.org/jakarta-commons/ValidatorExamples

Vlidation has good  integeration with struts.

I myself prefer to make some AJAX validation in this situation, it
makes better user  Interface and less page submits. I have seen some
samples of AJAX integreted in Struts-Layout UI components, which seems
to be easy to use.