I'm afraid it isn't that straight-forward. You could have routers A & B separated by, say, North America. Those two routers could operate at "full efficiency" (assuming the circuit is operating within specs) if you had a couple of hundred hosts all doing various udp, tcp, ftp, etc transactions. If you had two hosts trying to consume that entire pipe with a single tcp socket over that distance, you'd have some serious problems.
Characterizing traffic flows and network performance is as much an art as a science. There isn't really any one single magic show command that I'm aware of that will satisfy your curiosity. You can, of course, do basic 'sh int' several times to look at the load as a weighted 5 minute average (or run MRTG to get nice looking graphics). This will also alert you to any serious layer 2 errors that might be occurring on the circuit (SES, etc). You can also look at misc. stuff such as queuing, processor usage, and memory usage to ensure that the router isn't being killed. But none of these can allow you to sleep at night knowing that everything is at full efficiency. You have to get down and sniff individual flows, test throughput between hosts using ttcp and others, test response times and so on to get that level of comfort.
Having said all of that, if you have reason to suspect something fishy, I would recommend that you check your basic config. I once had an out-of-town client who insisted for several months that the SONET span between two DS3-equipped routers was magically providing timing. They even brought me out and put test sets on the circuit to prove it was running "error-free." Then I showed them the thousands of clock slips and unavailable seconds they were counting up along side that "zero bit errors" display. So we set one of the routers to generate clock and "performance," as measured by the number of user complaints, went up by orders of magnitude.
