If you are looking before the sale date, the clue is who is doing the suing. If the Plaintiff is a taxing entity like the county or city or school district it is a tax sale. If it is a mortgage company a bank it is a trustee sale.
If it is anything other than a taxing entity it is not a tax sale. It could be a judgment and still be sold by a sheriff or constable. It could be a trustee sale if sold by an individual who is owner financing and is foreclosing.