How can I tell from Activity Monitor if I need more RAM?

About OS X Memory Management and Usage

Using Activity Monitor to read System Memory & determine how much RAM is used

OS X Mavericks- About Activity Monitor

Understanding top output in the Terminal

The amount of available RAM for applications is the sum of Free RAM and Inactive RAM. This will change as applications are opened and closed or change from active to inactive status. The Swap figure represents an estimate of the total amount of swap space required for VM if used, but does not necessarily indicate the actual size of the existing swap file. If you are really in need of more RAM that would be indicated by how frequently the system uses VM. If you open the Terminal and run the top command at the prompt you will find information reported on Pageins () and Pageouts (). Pageouts () is the important figure. If the value in the parentheses is 0 (zero) then OS X is not making instantaneous use of VM which means you have adequate physical RAM for the system with the applications you have loaded. If the figure in parentheses is running positive and your hard drive is constantly being used (thrashing) then you need more physical RAM.

Adding RAM only makes it possible to run more programs concurrently. It doesn't speed up the computer nor make games run faster. What it can do is prevent the system from having to use disk-based VM when it runs out of RAM because you are trying to run too many applications concurrently or using applications that are extremely RAM dependent. It will improve the performance of applications that run mostly in RAM or when loading programs.

In your case one possible interpretation of the little information you've posted is that the computer is using swap space. If the use of swap continues to increase then you are running out of RAM. You are also using compressed RAM which does not mean you must add more RAM, but like swap space it is an indicator. Ideally, you'd like these to be 0 except under rare heavy usage conditions.

They have changed the names to "swapins" and "swapouts." Mean the same thing. What you want to look at is the number in parentheses for the "swapouts" report. In your posted report it appears that you are not having memory problems. However, earlier in the report you will find that you only have 95 MBs of unused memory. When free memory gets this low the system can begin slowing down, but this isn't the result of running out of memory as yet, but the system may soon run out and start using virtual memory. This is using the hard drive to provide memory because the memory demands cannot be met by physical memory. This is where things really grind to a halt.

In a 4 GB memory system that runs like this most all of the time I would say can benefit from adding more RAM.

You don't need more RAM if memory pressure remains in the green zone while you are experiencing a slowdown.

Already been stated had you bothered to read.