The easiest way to determine a Brass tube fitting is by the insert.

All tube fittings come pre-assembled with a fixed insert in the body of the fitting.

There are other fittings that have similar inserts however they are not brass. The inserts that are assembled in the DOT, which we will cover later, are stainless steel. They are also longer than the brass inserts for tube fittings.

Another easy way to determine you are working with a tube fitting or not are by the Nuts. It will have a plastic insert.

Brass tube fittings can be used with copper, aluminum, plastic, or nylon tubing. The sleeves for Copper, aluminum, and nylon require brass sleeves inserted in the nuts.