Hard Enough. Wide Enough. Safe Enough.
A good trail is Hard Enough, that the dogs do not have any punch through on the snow. If there is a crust overtop, and yourdog hits the crust, while pulling forward, they are likely to sustain damage to their shoulders or legs. A preventative injury, which could cost you the season. Look for trails that are hard packed by walkers, snow machines, or groomers.
A good trail is Wide Enough, that you can get your skate on, run your dogs past any other people or teams and has trees far enough away that you don't end up dead. People whose dogs stop to smell the roses, will also be happy with trees that are too far off the trail to prove tempting to pee sniffers.
A good trail is Safe Enough. Think of things that can endanger your team, and yourself. Know the area. Are there hills? Thin spots of ice? Lose dogs? Traffic? If you haven't skijored the trail before, ask around first or ski it without so you know what to expect.