You can check the memory usage on Debian Linux using the free command. This command displays the total amount of free and used physical and swap memory in the system, as well as the buffers used by the kernel. Here is an example:
$ free -h
total used free shared buff/cache available
Mem: 7.8G 1.3G 4.3G 140M 2.2G 6.3G
Swap: 2.0G 0B 2.0G
The output shows the total amount of memory and swap space, how much of each is being used, and how much is available. The -h option is used to display the memory usage in human-readable format.