I don't know how to determine RAM usage after the fact.
You can monitor usage in real time using Task Manager > Performance tab > Resource Monitor. But you need to catch the leak "in the act".
You can create your own log pretty easily as follows:
This logs the following values every 6 minutes (10x per hour): Timestamp, SoftwareUsedRAM, Uptime, RAM_Pct, Tag. A new file is created monthly.
I use a custom web app to visualize the data, but you can use Excel to import, view, and chart the resulting CSV data files.