eg M3? how can we see precisely how it is defined, measured?
The bottom of these indicator/series pages on FRED always lists the source. In this case, it's the OECD. If you dig around a bit on their site, and find where they host the data, there is always some meta-data attached to these that explains pretty much everything you would be looking for normally. Otherwise, they also list an email and you can ask them directly--which I've done on occasion. They're slow to respond, but get back.
For your specific question, for example, if you go to https://stats.oecd.org/, click on the 'Finance', 'Monthly Financial Statistics', 'Monthly Monetary and Financial Statistics' and the select 'Monetary Aggregates - Broad Money M3', it pulls up all the M3 data. On the right-hand side, there is a window you can expand that has all the metadata that answers your question.