There are several things you can do to insure that the fields are filled out, but you can't really tell if the fields contain real information or just garbage. Even the email field checking can only test for the proper address form, but it can't tell you whether the address is any good.
But the methods you use depend heavily on what you know and what you have available. Most web hosts provide a rather generic CGI or PHP form processor to their paid users. These usually have a minimum of checking, and are just designed to move field data to your email address. You usually do not have access to modify these scripts. So, if these form processors do not provide you with enough utility, you can upload your own form processor.
For example, you can get a nice PHP based form processor named DynaForm from www.webligo.com/projects.php. Just go to the bottom of the linked page, under "Freeware". Installation instructions are very clear. DynaForm can handle an unlimited number of forms and fields and includes a autoresponder, required fields, domain security, etc... The features are detailed in the documentation.
But that is not the only free PHP script available. A google search will turn up many more. You just have to go through them to see which one provides you with the most utility. You can even find ones that provide you with CAPTCHA capabilities. However, those scripts usually require that the host have some extra graphic utilities available. I don't know if Bravenet offer the extra packages.
A simpler method to check your fields would be to add some Javascript. An example of a script to check a email field can be seen at dynamicdrive.com/dynamicindex16/emailvalidate.htm. This example modifies the tag to include a call to a Javascript validation script. Nothing is submitted until a reasonable email address is entered. It's not perfect, but does a pretty good job. To check for other fields you would need to add more checking. As with the PHP scripts, a google search with the string "form field validation script" will provide you with other possibilities.
The problem with Javascript solutions is that, many spam operations run with Javascript disabled. In that case, the form will be processed without any checking. But it's possible to detect whether a browser is disable or not. In this case you would create your form so that it is, by default, disabled. If Javascript is enabled, it would be enabled dynamically when the page loads.
So, it's all up to you to determine how much effort you want to put into your form.
