As BGElect says, not easy, and it's usually a trial and error thing.
Adding weight to suspect items can change or get rid of the noise and identify the culprit. An alternative is adding tension to do the same.
For example, if the mudguard and stays are suspect, a piece of steel of any sort can be taped onto a stay or the mudguard. Alternatively, a bungy can hooked onto the mudguard or a stay and stretched to a frame point to apply tension. If the sound changes or disappears, you are on the right track to the source.
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