How can find out about the webpages with errors. Is there any good tool for it? Instead of delivering the website and than getting issues in the feedback, its better to check to check all the webpages with errors with some tool? What do you suggest?
Try this for your HTML:
The W3C Markup Validation Service
and this for your CSS:
The W3C CSS Validation Service
Well, hopefully if you are launching a somewhat big site you are using a template system. You can and should use the W3C Validator to check for errors. Having errors in the code makes the browsers guess what you are attempting to do which could make the page display oddly. This could lead to frustration on your part trying to compinsate for that.
[edit: was typing this at the same time Ralph was]
Thanks Mate, in fact both of you 
I appreciate your help. I am feeling like I am in heaven, whenever I have a question and i get a immediate and VERY GOOD response. I am loving this community. Thanks guys!
I like using the Firefox “Hml Validator 0.8.6.1” Addon because immediately after displaying a web-page in Firefox an icon appears in the bottom right hand corner. The icon is either red,green or yellow indicating errors, no errors or warnings. A popup appears when the icon is clicked which shows three panel as you can see from the attached image.
The particular page has errors showing that <h2>…</h2> needs enclosing in <li>…</li> tags.
The source and a help is shown in the other two panels.