How to isolate that network is issue of bad query performance
Waits and Queues is a good resource for this problem. Is a methodology for investigating SQL Server performance problems. It will teach you and help you find the bottleneck, which is better than to make assumptions and try to prove your assumption. Anyway, the paper has specific advice on how to identify Network as a bottleneck. A quick check is ASYNC_NETWORK_IO wait type stats, high values indicate frequent stalls waiting for Network IO sent to the client to complete (ie. marshaling the results is slow). Read the paper for more details.

Note that well designed applications seldom are affected by network bandwith. Short of special ETL scenarions, a high bandwith requirement is an indication of bad app design, not of network problems. High Network latency also affects chatty applications more, and chattiness (many round-trips) is again a smell of bad design. Both network badnwith and latency are trivial to test and measure, again read the paper linked for methodology.