The memory usage bar, located under the Sources panel at the bottom left of your screen, shows how much RAM Geneious has available and how much it is using. This can be turned on in the Preferences → Appearance and Behavior tab.
Note that Geneious runs inside a Java Virtual Machine, which itself will use memory. Looking at JVM usage under Task Manager on Windows, or Activity Monitor on a Mac thus won’t give an accurate measure of how much memory Geneious itself is using, as it is the total RAM allocated to the JVM that users will see from the various monitors.
Note that the Geneious memory usage bar doesn’t include the memory used by external plugins such as the PAUP and PHYML plugins, as these run outside of Geneious.