You can do all the checking you like, but it’s quite likely that you will never find all the glitches that your users will find.
There are a number of ways you can keep up to date with issues with your site. I’ve added a list of some easy and cheap methods:
Track your error pages
Google Analytics is a free option, great for small businesses. If you modify your code on your error pages, you can check regularly for broken links and problem pages (these may even be broken links from other sites or links typed in by users).
With this knowledge, you can fix issues with your site, request that other sites linking to you update their links, or create some custom content that people seem to want to find.
Here’s a link to the instructions, Keemo has written up.
Check out your goal funnel
Here you can identify issues (not necessarily just errors), with your conversion path. This may uncover a particular problem page, which puts users off buying or signing up.
If you’re using Google Analytics, you can set this up quite easily by changing the settings and adding each url used for a purchase process. Other Analytics packages allow you to do this also.
Now, about that 10 page checkout process…
Monitor your server status
If you’ve got an advertising campaign on the go, it’s important to check for server outages. Sign up to something like Monatastic, which will send you an email every time you need to pause your advertising and kick the server.
Ask your users
You can install a couple of feedback tools which can ask your users about their experience of the site, or view what they do. Use ClickTale for videoing your users path, Google Talk Chatback to let them ask questions or talk to you, and 4Q to ask them how their experience was once they have finished.
More fantastic tips on fixing usability issues with your website here: Conversion Rate Experts.
Get someone else to look at it. Ask them to complete a set of tasks, and talk to them while they’re doing it. When you’ve designed a process, it’s obviously logical to you, but is it logical to everyone else?