While a hang glider can be flown safely and effectively with no instruments of any kind, many soaring pilots choose to use an instrument panel, which is known as a vario, which is short for "Variometer".
The simplest type of vario is an audible only device that attaches to your helmet. One type of beep means that you are going up, another type means that you are going down.
A step up from this is the simpler version of varios that have displays. These typically have one or more altimeters in addition to a rate of climb/rate of sink function. The newer varios in this class include flight logging, and there are versions of these that have the ability to download the logged information to a computer. These instruments can be fitted with an airspeed indicator.
There are also varios that can accept input from a GPS unit, and varios that have an integrated GPS unit. These can log you flight in both altitude and location, and you can download the information and replay it on your computer. If you have a GPS enabled unit and have fitted it with an airspeed indicator, it can tell you your airspeed, your groundspeed, and the wind velocity and direction. Not surprisingly, all this comes at a price. The combination of GPS enabled vario, airspeed indicator, and bracket will cost more than $1000 US.
A simpler approach to airspeed indication is the Hall airspeed indicator
The airspeed indicator and a bracket can be had for $30 US.
You don't need a vario to fly a hang glider. On his personal site, Red has posted an article on judging your rate of climb or descent without any instruments: Thermalling Without Instruments