Form validation?

For a new site I use form_mail.module and forms.module to create several different mail / contact forms, that can be used by anyone (it's not necessary to log in as a user) and that will be sent to different addresses (depending on the form). These addresses are hidden BTW.

The forms are created by using the required php code at a static pages. At submission of any form, I'd like to validate some fields. Think about fields that are required, or fields that contain an e-mail address.

What is the best way to achieve this: how should I validate each form? And can anyone give me some advice about how to avoid spam with these type of forms?

Thanks for replies!
Marc