The biggest sign of low RAM is when Windows alerts you with a message something to the effect of "Virtual Memory Low". This happens when the apps that you're running demand more memory than can be accessed.
Another sign is when performance is sluggish. Meaning apps can run, but the overall speed of the system seems to have slowed down.
Low RAM (or as you asked, the signs of low RAM) differ from the lack of CPU "speed" (not power, as "power" refers to the energy it uses) in the sense that the CPU process info that is sent by RAM.
Let's simplify this...
With only your mobo, cpu/heatsink, and power supply installed, your system can power up; however there is no display. The reason the system has no display is because there is no instruction to the CPU, meaning nothing is "telling" the CPU to "process" anything.
So you install RAM, and now you have your display (provided that you have onboard VGA and the monitor connected, of course). You now have a display, because the BIOS tells the RAM to tell the CPU to communicate with your GPU or onboard VGA. With your display finally up, you'll notice either of two things:
1. A splash screen, instructing you to press a particular key to enter "Setup" or "BIOS"; or
2. A black screen with white text informing you that "no bootable device attached..."
This is because there is no instruction set. The instruction set is stored on the hard drive. So you install your hard drive and now you can load into your Operating System.
The hard drive has the information that is sent to the RAM for temp storage and later processing by the CPU. This is done because no hard drive can't communicate fast enough with the CPU; it's like the two parts speak a different language. The RAM is an intermediary.
To sum it all up...
The hdd sends the info that needs "processing" to the RAM for usage by the CPU. Since the processing happens in this order, the bottlenecking occurs at the memory controller, which is integrated in the CPU. What this means is, if the RAM isn't fast enough to keep up with the CPU, the CPU will halt while the RAM finishes sending info/data to it. When this happens, you experience sluggishness.
Why don't you post your system specs and the problem you're concerned with so the culprit can be pinpointed.