The reason for spooning a fish is to find out what insects it has been feeding on so you can then select an appropriate pattern to imitate the naturals.
Bear in mind that if the contents have been in the trout’s stomach for some time they will have started to break down and probably be completely unrecognisable. But if the trout has eaten quite recently you should be able to recognise such things as buzzers and nymphs, and if they are still alive maybe even identify their actual colour.
The tool used for this task is called a marrow spoon, available either as a combination priest/marrow spoon or a separate instrument.
Only spoon a dead fish – you wouldn’t believe some of the things I used to see in my time running fisheries and yes, one of them was someone trying to shove a marrow spoon down the gullet of a live fish!
When you have retrieved the spoon from the trout’s stomach the contents at the very tip of the spoon will be those that have been in there the longest, and probably the most unrecognisable. In the middle will be semi-digested insects, but still worth imitating, and finally the contents nearest the handle will be its most recent meal – and probably the easiest to identify.