This can be very easy or long and complicated.
If you don’t know where your solenoids are then you will either need to dig around in the more obvious places (garden beds, start of lawn etc) or you will need to use a solenoid detector.
Sometimes this is the only way to find that missing item. When a house has been remodelled and the yard redone it can become even harder and the solenoid detector is not foolproof.
Today I found myself hunting for a hidden solenoid and without the detector it would have been a complete waste of time. However with it I was able to trace the wires, dig a hole 600 ml deep and find the offending valve.
If you need to locate a solenoid then I don’t promise we will get it, but in 4 years only one has beaten me, so its a fair chance we will be able to locate it!