How to Detect Soft 404 Errors on my site, except checking on Google Webmaster?

I need to check on all the Soft 404 errors on my site before Google locates them. Is there a way to do it? any tool? anything? any tactful idea?

 

you could always scan and check for 404's in the access logs.

 

Just run a custom 404 page that logs it for you say redirect any 404s to a generic error page and log the requested url to a database

.htaccess
ErrorDocument 404 /error_404.php

Code (markup):

 

You could build a robot script to link check your site.