General:
During engine operation there is a strong low pressure (suction) force in the area in front of the engine inlet. Obviously you should stay out of that area and make sure there are no objects that an cause FOD to the engine, securing stands is also included.

Something to think about during engine operation is there are toxic gases in the area behind the engine. These are hot and have high speed. Again it's common sense to stay out of these areas and also there area no items in that area that can be damaged.

The noise level is also high so wear ear protection, obey time distance restrictions.

There are some other basic things to think about too:
Ignition should be off when you perform any maintenance operations that do not require it to be on.
Be aware of hot engine parts. Pneumatic tubes and attached components, Fuel and Oil Tubes and Components, Combustion and Exhaust sections.

Wind can affect good test results to get consistent reading the aircraft should point towards the wind. The wind speed should be less than 25 knots. The wind direction is not important wind speeds less than 8 Knots or blast fence is used.