One of the easiest ways to get a quick check and see how your html is doing is to get a "view source" of a page, copy it into your favorite text editor and count the <table </table, <tr </tr, and <td </td tag and see if the numbers match. If they are unequal between the opening and closing tags, then you know you have some html problems. Netscape 4x is pretty unforgiving in these situations and you'll need to go in and see where the problem is and trace it back to your hacks that have been added.
If you have been told a particular page that has been giving them problems, post a link here and I'll help you go through it... it can be a tedious process, but your site will load faster and look better in all browsers once you've done the process.
Another way that might save you a little time is paste the url of a particularily problemsome page over at
http://validator.w3.org/ - it wil help you pinpoint the areas that have problems.