You can do this under the Carrier Configuration in System->Configuration->Shipping Methods
Switch debug to Yes in the particular carrier you need.
Then under Developer->Log Settings set Enabled to Yes.
You can now view the logs in the file system under var/log.
See also this screencast on WebShopApps specific logging.
The error messages are very descriptive if no rates are returned from the carrier. Errors will be displayed at the end of the log.
The following examples illustrate what happens when a product in the cart is over the maximum weight.