Although many amps tolerate no load well, you can never make generalizations because that is not always guaranteed. A proper power resistor across the output of the unloaded channel is always a good idea. Perhaps more importantly, you are testing the amplifier (or at least, using the amplifier to perform a test on another component). This implies you will be driving the amp with "something".
Obtain two power resistors and use them on any unloaded (not under test) channel. One per channel, 8 ohms is always a good value to use as a general rule. They will get hot ... air cooling is usually fine but not always enough. Don't touch them during or after power has been applied; burns are possible.
The power rating of the resistors is not really easy to give general answers on but if you limit testing time you may be OK. More is always better with regard to the power rating, but of course there is a cost penalty as the rating goes up. I would consider 10W as a minimum but if you can find higher rated units, get them.
It's the only way to be reasonably safe. Most people would build a test fixture to house the resistors, but if you're careful it's possible to use them naked. Re-read the part about getting hot, above.
There is often no problem with shorting inputs as Scotty suggested, but again, it's hard to generalize because this is not guaranteed in every possible configuration. Care should be taken if you don't know for sure. And a final warning ... NEVER short outputs, only inputs.
Remember the cardinal rule about electronics: components are like Aladin's Lamp, and if you rub the lamp the wrong way, the smoke will be released. It is always your goal to keep the smoke inside the lamp.
