How can I estimate how much humidity is present in a NaOH granulate?
For a rough estimate, you could do the following:


Weight a sample of it and heat it above 100°C for a while (say, one hour?).
Weight the sample afterwards.


The difference should be the amount of water given away and is an estimate (from below) of how much water there was in the sample.

But be aware that you may have some sodium carbonate depending on storage conditions.