There are hundreds of different functions you could check. On most sites a good starting point is the following basic checks.
- Test your site in different browsers. As a minimum, check with Firefox, Chrome, Safari and Internet Explorer. You may find you have some errors that only effect IE. Search the Internet for solutions, you will not be the first to have the problem.
- Check the load speed of your site. The index (home) page is most important. Using a page speed tester (again search the Internet for suitable tools) will show the speed of your page. You will usually find images are larger than they need to be and so can be optimised for faster loading. Some scripts may be larger than necessary. These can be optimised to increase page load speed.