how to find the problem behind a broken page

this is just a general question, but how would i go about finding the problem behind a broken page?

for example, my search page is all screwed up. when i search for something, the layout completely breaks down, even though i believe it has the code to work.

anywho, i use firebug etc. which is great for finding css errors, but is there a way to go about finding problems in page templates, and php files?

thanks!