Making sorting discoverable on a table with no default sort

We have a table that orders results of clients (can be searched on name, address, email, phone, etc.) based on an algorithm, so there isn't a "default" sort we can mark with an arrow to indicate the table is sortable. However, there are two columns that are sortable, but you can only see a sort arrow indicator after you discover that and sort one of them. What is the best way to make this discoverable?