One such way that bosses can do this is by looking at your social media sites such as facebook, twitter, instagram... the list goes on. Many employers do online checks of sites like those before even giving you your first interview. as such it is very important to make sure that you are using those services responsibly and presenting your self in the best light possible. Because if you have material on a profile of yours that could paint you in a negative light and a prospective employer sees it, they may not even give you the interview.
Now, that's not to say that social networking sites are taboo and that having one will prevent you from being hire-able. You just need to make sure to give yourself the right online image. These sites can be used to show you in a good light so that employers that look you up will actually be impressed. Plus these sites give you an extremely easy way of staying in touch with friends and family, especially those living far away. As a college student I use my facebook account to stay close to friends from high school who I don't get to see very often. While it is important to be careful about what you post when on social media sites, they can be used as a helpful tool to keep in touch with others and even make you look more desirable to a potential employer.