Step 1 - Validate the code 
W3C® Validator
— file upload version
— uri version
— text entry version
Unofficial W3C CSS Validator Faq  new - small - mostly access problems for now
HtmlHelp.org - CSS1 Validator

 Step 2   find out if it's supported in any browsers
Style Master  style editor, great tutorial 
— css1 css2 compliance charts  includes e6 opera and webtv
— border margin padding chart
Jesset.com - Compliance Chart has opera/webTv
Mastergrid - Css Support Charts (no e6)  
PPK css2 tests  explorer 6 Opera and others included
CSS Pointers Group  large list of known bugs
HtmlHelp.com CSS check form
MacEdition.com - MAC browser chart
— osx browsers
— css3 selectors chart

 Step 3  See how it actually looks done right - if its the same as yours, look for a workaround.
W3C® ORG  css1 test suite - correct use is assumed here
— w3c float test page
— w3c visual formating model
Cwru.edu - CSS2 Test Suite Prototype — Another Version
Mozilla.org - Tier 1 Tests
Dave Baron's Browser Test Results - The Tests and his Playground
Eviltest & Wetblanket Test  So your browser passes the CSS1 Test Suite and you think it follows HTML closely too? Try putting it through these tests