EGT measuring

how do you measure the exhaust gas temp on dyno?

Engine Dyno's often have a dyno header with an EGT sensor mounted for each cylinder. They dyno logging software will have a provision for displaying and logging this information.

Otherwise, you could fit EGT sensors when the engine is installed in the car. This would typically be connected to multi-channel EGT amplifier and connected to either the ECU or logger in the vehicle.

On topic. I have EGT probes mounted one inch from the head on each cylinder that report to a bar graphic gauge mounted in the car. My question is what is a safe temperature at full throttle under a hard dyno pull ?

Unfortunately it's not as simple as having a clear cut temp that you must not exceed. EGT is affected by many parameters including the fuel you're running, FI or N/A, your ignition timing, and the type of EGT sensor you're using (enclosed vs exposed element). This makes it all but impossible for us to give you a firm answer. All I can add is that in turbo applications I tend to try for an EGT below 1000 deg C to protect the turbo from damage due to sustained high temps.