Just as a business has assets, liabilities, and equity; your personal life has similar accounts. Your net worth is a calculation of your equity, that is your assets minus your liabilities.
Determine your assets. Assets are defined as anything of value. In regards to retirement, your assets typically include equity in your home, a 401k or similar account, any IRAs, checking and savings accounts, anticipated social security benefits, and anything you may anticipate using to fund your retirement.
Determine your liabilities. Liabilities are anything for which you are indebted. For retirement purposes do not attempt to measure your monthly bills simply the big picture items for which you are in debt. These typically include your mortgage, your vehicles (which may actually have equity in them), credit card bills, and loans (home equity, student, etc.).
Now you can simply take the sum of your assets and subtract the sum of your liabilities to calculate your net worth. A bar graph is an effective way to monitor your net worth from month to month.
Understand that your net worth is merely a number. No one can tell you when you have enough to retire. If the largest portion of your retirement assets is home equity are you willing to downsize to tap that equity? Will your retirement be a quiet, inexpensive home in the midwest or be a travel filling romp around the world? These are questions only you can answer.
It is burdensome to manually calculate your net worth. A site that makes this easy is Yodlee MoneyCenter. This site also provides service for major banks to allow you to access their site through your bank. For example, Bank of America Portfolio feature is run with Yodlee technology. To utilize this site, simply list all of your major accounts and your usernames and passwords and the site will retrieve your information from each site and calculate your net worth.