According to the documentation, the ENGINE CONTROL SELECT SimVar is a settable bitmask.
This implies you would use this to inform the Sim which engine(s) you are about to control or query.
Selected engines (combination of bit flags):
1 = Engine 1
2 = Engine 2
4 = Engine 3
8 = Engine 4
To determine if an engine is running, the documentation suggests you should use ENG COMBUSTION (True if the engine is running).
Presumably you would request this after selecting the engine(s) you wish to query.