Anything that happens on my side of the court, I will be 100% honest about and give benefit of the doubt.
Anything on the opponent's side of the court is their business. I might think my serve went long, and most of the time if I ask, they will say it was good. I just stopped asking. If they don't call it, I'm playing it. If they are unsure about a call and ask me, chances are I will tell them what I really think. But it's their responsibility.
If it's not an official match, I'll make difficult line calls for them if they couldn't see it, though I feel it's sometimes bad mannered to do it during a serve, since they will always have a better point of view except on the center line, where you will have just as good a view, if not a better view, of the line.