There really isn't another piece of test equipment that will tell you if you are driving the amp to hard. Yes clipping will vary with the ambient temperature, like i said earlier, the whole point is to keep the transistors cool.. search the online pdf version of your receiver, there may be a "clipping" indicator. On the amp i have in the bedroom, each channel has a clipping LED indicator, that will light up if a channel is being driven into the clipping region, i don't know if this is an option that receivers have or not.
When i would tell you to set the max volume maybe 5-10% higher than your "reference" level, which for you would be your movie level. See how everything works at the max setting, and go from there, But keep in mind, if your wife has another party, and one of her friends does turn it up, they may reach the max setting, and the system may be left there for an extended period of time, so keep this in mind.
Unfortunately, this is one of those topics that is a grey area, there is no quantitative answer that we can provide you. One option would be to call the manufacture, and talk to a technician, they will have MUCH more information on what a safe level for extended use would be.
