Checking to see if someone else is using my EIN

I don't think there's any way to know about accounts that are (only) opened, or only used for non-income purposes.

If there are accounts that produce potentially taxable income above a threshold, those will be reported to IRS on 1099-series forms shortly after the end of the calendar year -- e.g. for 2021 in February or March 2022. After those 1099's are processed and posted, which may take another month or two, they should appear on a 'wage and income transcript' which you can get for free. You will probably need to use the paper/mail method; AFAICS the online methods only handle individual filers not businesses (and only a fraction of them, according to frequent NTA complaints about the problems people have passing the authentication for IRS' online apps)

You can monitor your business credit like your consumer credit. Unlike with www.AnnualCreditReport.com for consumers, though, business credit monitoring is not a free service. Experian wants $189/yr for their "Business Credit Advantage" service.

As noted in the comments, an EIN is not a secret from anyone. But if you're concerned someone is using your EIN to commit fraud (or you suspect they might) then I would call Dun & Bradstreet, the biggest reporter of business credit, and ask what can be done to monitor your EIN for activity. You can do the same with Experian and Equifax, the other two big reporters of credit, and you can also try CreditSafe. You can subscribe to services any of these companies offer to monitor your credit, but you may not be told WHO is looking at your credit, just that an inquiry has been made.

The biggest issue you have here is that unlike with personal credit, the majority of companies that extend trade credit don't necessarily report it. According to [this article on Motley Fool][1], only around 3% of business trade lines are reported to a bureau. So the real problem is that if a vendor gives a trade line and doesn't report it then you'd never know about it unless the debt goes bad and the vendor comes looking for their money.

Now, something else to keep in mind - the federal government DOES require creditors to collect some personal information about everyone involved with the business who owns 25% or more of it (they want name, address, SSN and DOB for each one), and it's doubtful your employee would have that information to give. If your employee tries to open a business Visa/MC account, the rule of thumb is that the banks will require a personal guarantee (which requires a credit pull on the guarantor) unless your company meets a very narrow list of criteria that doesn't require such a guarantee, but most likely the guarantee will be needed.

I realize you're concerned, and you should be, but don't let it drive you crazy. If your ex-employee DOES open accounts, they're going to have to provide information (for billing and shipping addresses) that will make it easier to detect and find them, or since what they give doesn't match what's on file for your company with the bureaus, any such attempts might be denied altogether.

Keep a careful eye on things for a while, and it wouldn't hurt to contact your existing creditors to notify them of the issue so they can take appropriate measures to safeguard your account and be aware of any improper attempts to make changes to those accounts.