There is nothing more stressful than owing the government more than you can afford to pay. Lots of people in this situation stop answering the phone and opening their mail. This is common and understandable. However, you need to know how much you owe to get an Offer in Compromise approved.
The easiest way is to check online. The IRS has a website where you can see what is on your account. Simply go to this website: https://www.irs.gov/payments/view-your-tax-account.
From my experience, one in five people will not be able to verify their identity for this tool. The next easiest way to do this is to open your mail. Around the first of the month and right after you file your taxes, the IRS sends their collections letters for each year you owe on. This will give you a good sense of the total amount expected by the IRS.
If all this fails, you will need to call the IRS. No one likes to call the IRS but the collections people are actually pretty reasonable as a group. They also cannot force you to do anything and they have to answer your question.
All you need to do is call 1-800-829-7650. The hold time is generally pretty light here since the IRS wants its money. When someone answers, ask for a compliance check on you and your spouse’s account if you are married. The agent will tell you what years you haven’t filed and what you owe for each year after you verify your identity.