How can I tell which of the available networks has the strongest signal?
When you do a site survey you will see a column for signal strength in the results:
The negative numbers in brackets are the ones to look at (this is the signal strength in dBm), note that these are negative numbers so -33 is a higher (better) signal than -73, because -73 is a much lower number.
In the example above the second entry at -33 has the strongest signal.
How much signal strength do you need?
For simple, low-throughput things like sending emails, browsing the web as low as -70 should be OK, towards -80 things may start to become unreliable and approaching -90 it is unlikely to work at all. For higher-throughput applications like voice over IP (eg. Skype) or streaming video -67 dBm or higher is better.
Is there a simple rule of thumb?
Basically if you have a choice of networks that you can connect to (ie. unsecured or ones you have a password for) then choose the one with the signal strength number closest to zero.