Unfortunately, unless they are specifically mining at NF=16, there's not really a great gauge. Because of the memory requirements of the coin, not all of the shaders can be utilized, so the card is running at a portion of it's full capabilities. The best way to tell is to plug your system into a kill-a-watt or similar device and take an idle reading of the watts, and then take a reading while running various mining configurations. You may find that a slightly lower hashrate (perhaps from a lower lookup-gap) yields a higher hash-per-watt, and that may be of interest to some people. I haven't taken wattage readings since NF=14, but I was measuring about 25W per R7 240 at that time, and ~32W per R7 250.