Although the network, 3G, and 4G icons that reside in the HTC EVO's status bar provide a general indication of signal strength, they can often be the cause of frustration when you're not seeing the kind of performance you might expect from a "full bar" signal. There are many factors affecting speed/performance/etc. when you supposedly have a full signal, but one way to manage your expectations is to know the actual signal strength in terms of an RSSI measurement.
There are several apps out there that let you view WiFi signal strength (e.g., WiFi Analyzer), but did you know that you can view RSSI measurements of the EVO's network/cell, 3G, and 4G radios right out of the box? All the information (plus some other details) can be found in a hidden setting called Field Trial.
To access it, just dial ##debug# on your phone. You'll see the screen shown above (that's the 7" dual-screened Toshiba Libretto W100 behind the EVO in the photo) almost as soon as you lift your finger from the final #.
Tap on "1X Engineering" to view 1xRTT info, "EVDO Engineering" for 3G info, and "WiMAX Engineering" for 4G info.
What you're looking for in the list that will appear is "RX Power," which is the RSSI. You'll see a number between around -100 dbm and -50 dbm. The closer to -50 dbm you are, the stronger your signal is. I believe -50 dbm is considered to be a full signal. The best I have seen on my EVO is -65 dbm, though even at -72 dbm my status bar icon still shows full strength.