You can never really be sure, but you certainly need to make sure you work with the right professionals including a small business attorney and business broker and possibly CPA, and you also need to make sure you get valid information (your professionals will help you make sure the information is valid, using deal terms like disclosure and so forth, that will put teeth into disclosure. That information should include detailed past financial results with some weight of law like actual tax forms, details on assets and liabilities, and of course, details on actual sales, costs, and expenses.
Having all that, you've just started. You need to compare this business to others like it, just like when buying a house you compare similar deals in houses. It's not simple but it's very important.
Next step: find the right attorney and broker. Get help.