The first step is to find out if the neck is as straight as it should be and if not what direction it is bowed in. People who do this all the time generally can tell just by looking. The best way to check visually is to look down the side of the neck from the peghead end. On a banjo this is much easier to do on the side by the 1 st string. The trick is to separate the line of the neck from the line of the strings. Strings are straight of course, but usually there is some bow to the neck. I often put my left hand on the bottom of the neck to cut off the sight of all the strings and the head. This makes it easier to focus on just the neck. In the perfect world there should be a very slight curve up from the bottom of the neck. This helps keep the strings from buzzing. This curve should be very gentle.

Another way to check is to use a straight edge on top of the frets. I made one using a piece of aluminum stock I bought at a local hardware store. They come in various lengths and widths. Mine started as a piece 1/8” x 11/2” x 48”. I cut it down with a hack saw to about 24 ¾ inches. It should be long enough to run from the nut to the end of the fingerboard or a little beyond. To use it put the banjo on a flat surface. You should put something under the bottom of the neck close to the rim. If the peghead is holding the neck up it will bend the neck.

You can also try putting the resonator part of the banjo on a table or counter with the neck hanging over the edge. This only works if the banjo pot is heavy enough to keep the banjo from falling off (keep a hand on it at all times). Put the metal bar on the neck between either the 2nd and 3rd or the 3rd and 4th strings with the thin side on the frets and one end touching the nut. If there is an obvious gap between the bar and the frets in the middle of the fingerboard you know the truss rod will need to be tightened. If you can rock the neck on either end of the fingerboard you will need to be loosened. When the neck is good you should be able to slide a very thin piece of paper (I usually use sticky notes) between the bar and one of the frets in the middle of the neck.