Ya know this is a hot topic:
First I'll start by saying when I was a kid I just went there and fished and if people didn't like it the cops would show and I'd leave.
More recently in my calmer days, there have been a couple of situations that occured on the NF lewis and EF lewis. On the northfork there's a gentlemen that owns the rock on the northside of the meathole. During the spring it's usually closed to boats so the best spot to fish is off his rock. So we used to just go there and fish and he'd yell at us to get off the rock and we'd yell back about the high water mark and then one day he called the cops. Well the cop showed up asked us over and told us we couldn't fish there anymore. I asked the cop why and explained about the high water mark. He said he didn't know the law and said that we just had to leave.
On another note we were fishing the EF of the lewis and there's a hole above daybreak bridge that rocks for winter runs. We'd always anchor there during the weekdays and on the weekends the guy would come out and yell about anchoring in that hole, claiming he owned the river. That guy I told to pound sand and he's now moved and the issue is over.
But overall, the best way to go is to ask before you even enter the property. You'd be surprised how many people let you fish there land if you give them fish to eat, and keep things clean. Some people I've even offered to help with hay for the summer to gain access during primetimes.
It's sort of like hunting ask and thou shall be granted access!
Keith