BullseyeBore is a disk-shaped tool that attaches onto any drill and it shoots out concentric circles to the surface. While the two inner circles remain fixed, the outermost circle will move dramatically even if the drill is misaligned by a few degrees as a signal that the drill is not straight.
However, the tool is not available for individual purchase as the inventor only accepts companies and retailers.
There are three methods on how to drill a straight hole demonstrated in this video.
The first method demonstrated uses a doweling jig (see image above), a tool intended for drilling holes on the edge of a woodwork. But with a little improvisation, it can be used as a guide to drill a straight hole on the surface.
The second method uses a woodworking drilling locater (see image above). Not only it can drill a straight hole on flat surfaces but it can also drill a straight hole on the edges, on cylinders and on spheres. Unfortunately, unlike the doweling jig which features six drill hole sizes (3/16″, 1/4″, 5/16″, 3/8″, 7/16″, and 1/2″), this only features three (6mm, 8mm and 10mm).
And the third method is a DIY version (see image above). Basically it’s two normal blocks of wood glued or nailed together to form a right angle and the inner corner is used as the guide. As simple as it sounds compared to the other two above, there are no limits on the size of drill that can be used with it.