In linux, it's easy to see your levels, use iwconfig when connected to wifi, and look at your Signal Level.  It'll be represented as a negative integer, say -50 dBm.  The higher the number (remember, this is negative), the better the connection.  For instance, Cisco recommends voice coverage isn't lower than -65dBm for voice quality on wifi clients, so I tend to use that as a watermark for good/bad, but usually -40 to -65 is acceptable, anything over -75 means you're too far away, or there is interference to keep the connection.

If your levels are good, and you're still dropping, I'd say look at the drivers and/or other causes.  Watching your syslog file at networkmanager output when connecting/disconnecting can be telling.

Also, if you're using 2.4ghz wifi (802.11b/g/n), then you're very susceptible to interference - microwaves, fluorescent ballasts, all sorts of things can cause EMI.  Better to use 5ghz, but also less range than 2.4ghz with the higher frequency.

There is a big difference in wifi dongles, usb ones in particular tend to be variably craptastic.  This was a good look at testing between different usb dongles as they use them for testing wifi in enterprises I found recently.  Long story short, try an Asus usb wifi nic, they seem to test best.

HTH!

-mb