All your static shocks have their charge generated by friction between two insulating materials (eg your clothes & car seat, shoe soles & carpet etc). Wintertime tends to be bad time for shocks due to the low air humidity levels, (air at or below 0C has zero humidity). When the humidity is high, the moisture provides enough of a conductive path to significantly reduce any static build up.
Shoes are available fitted with soles which prevent static build up (the sole material is partially conductive) � commonly used in the electronics industry to stop static destroying electronic parts handled by staff. It is possible to fit heal straps to normal shoes which have the same function � but this will make your Nikes look very un-cool.
When touching someone results in both receiving a shock, this is due to one person being charged � when they touch, the charge is �shared� between the two bodies, or the non-charged person being earthed, providing a discharge path. Becoming charged (as normally occurs), is not what causes a static shock � it is the sudden discharge which causes the shock.
There is no easy way of determining whether you have sufficient charge on your body to cause a shock.