1. Your bench or stool is adjusted, and/or your harp is on a box, kneebones or a strap, so that you can reach all the strings easily.
2. Your harp is the right distance away so that when it leans on your shoulder the strings are roughly perpendicular to the floor and your knees are taking the weight of the harp. (Or your small harp remains upright and you move it comfortably close to you).
3. You back is straight; your neck and shoulders are relaxed (no hunching).
4. With your arms outstretched, your hands comfortably reach the middle of strings.
5. Neither arm has to cross the midline of your body to reach the harp, because the harp is somewhat diagonal to you (not perpendicular).
Not sure? If you can find a mirror, sometimes it helps to look at yourself and analyze your harp posture from the outside. Or have a friend take your picture with a digital camera. The results may be revealing!
Finally, remember to move. No position is comfortable if it's inflexible. Holding tension anywhere is what creates issues in our bodies, so remember to keep breathing and think about being fluid as you play. If your position feels at all precarious or cramped, you will not be able to move freely, so this is the final test of a good working posture.