""Doctor, come quickly! Buffer Cache Hit Ratio is way down; Disk Queue Length is through the roof, Page Life Expectancy has dipped below the magic '300'. "
These are all Performance Monitor Counters and any experienced DBA will tell you the basic key to using them is not to over-react to them literally at any specific point in time. Otherwise, you are just going to be chasing wild geese all the time. The real secret to using them is to track them to a file over time and then examine the trends. SQL Nexus is just one tool that comes to mind that can help you with this. :-D
"Technology is a weird thing. It brings you great gifts with one hand, and it stabs you in the back with the other. ...:-D"