For native drivers and io-net drivers, the nicinfo utility is usually the first debug tool that you'll use (aside from ifconfig) when problems with networking occur. This will let you know whether or not the driver has properly negotiated at the link layer and whether or not it's sending and receiving packets.
Ensure that the slogger daemon is running, and then after the problem occurs, run the sloginfo utility to see if the driver has logged any diagnostic information. You can increase the amount of diagnostic information that a driver logs by specifying the verbose command-line option to the driver. Many drivers support various levels of verbosity; you might even try specifying verbose=10.
For ported NetBSD drivers that don't include nicinfo capabilities, you can use netstat -I iface to get very basic packet input / output information. Use ifconfig to get the basic device information. Use ifconfig -v to get more detailed information.