Slowly and methodically. One person on deck with a hose drenching each suspect fitting or joint in turn. One person in the cabin looking to see when the water comes in. I was convinced my stanchion bases were leaking until this method proved it was the foreward hatch.
Partition your deck off into 1 metre square areas. Use coloured dyes in fresh water in each seperate parts of your deck then check the colour of the drip. You may find that it is an amalgamation of two or more colours then you know you are into a complex situation which always helps before you start fault finding.
I would just wait until there has been five consecutive days of hot dry air with no rain - and then go around everywhere with sikaflex squeezing it into every gap and wiping off any excess with a solvent covered rag before it has solidified. This will leave a neat job and a dry cabin.