Most of the touchscreens that I have seen on Delta (until recently) have been of the resistive type. These are the kind that have two thin layers and register a touch where the layers are forced together. They are easy to identify when light hits them from an oblique angle - you will see a grid of dots. They also don't support multi-touch, or gestures (usually). These ones work well with the "fingernail" method described above. They are most accurate with a "point contact". Many of the older IFEs that I have seen, especially those ones with the screen about the size of a smartphone, are so far out of calibration that you can't even push some of the onscreen "buttons".

Capacitative touchscreens work by virtue of the electrical conductivity of your skin, which alters an electric field applied to the surface. These work best with a light fingertip pressure, and support things like "pinch and zoom", "drag" etc. Most of the newer IFEs - the ones that are 8" and larger, seem to be of this type.