The IRS is required to notify you before seizing your assets.  This means that you'll receive a Final Notice of Intent to Levy in the mail prior to receiving a bank levy or wage garnishment. 

A Final Notice of Intent to Levy (Notice LT11, Notice CP90, Letter 1058) gives the taxpayer Appeal rights.  You may request a Collection Due Process (CDP) Hearing if you disagree with the IRS' intent to take your assets as payment for the tax.  The IRS allows 30 days from the date of the notice to submit a CDP Appeal.  But you might want to get a professional opinion before you submit a CDP.

If you receive a Final Notice of Intent to Levy and Your Right to a Hearing notice from the IRS, it's about to get real ugly if you don't do something quick.