The probable cause is if he sees you fishing or sees evidence that you have been fishing. If you're carrying a fishing rod or a stringer of fish around in an area where you might be fishing, or have been fishing, (near a lake or stream) that's probably good enough. (Remember, you need a fishing license even if you're releasing everything you catch, or even if you haven't caught a single fish.)
In California, if you're a passenger on a sportfishing boat (even if you don't have a rod) you're required to have a license before you even step aboard.
If you're cited for this, if you have enough witnesses who are willing to go to court and swear that you never so much as touched a rod while the line was in the water, you may be able to get the charges dismissed, but that may be more trouble than it's worth. And if you did touch the rod while the line was in the water, even just for a second, that counts as fishing.