Howdy,
I live in an area where the situation you describe is common and can tell you with certainty that a road on onX is not necessary public. My home is on a very short private road that ends in an old national forest dozer grade. On onX it looks like you should able to turn onto my road and drive right into the Okanogan-Wenatchee NF. It is a private road however, and I would certainly come out to greet you and see how your day is going. The opposite is also seen here in many places: forest service roads pass right through private ranches or peoples yards as public rights of way. If a road sign is blue it is generally private. Gates without signage may or may not indicate private and closed access. Mostly they are just to keep cattle on grazing land.
If you knock on a door and say “My Apologies for bothering you, I just wanted to make sure I was not trespassing”...you may get a dirty look or you may get access. Just don’t do it on opening day holding a weapon

If it’s a numbered forest service road without a locked and signed no trespassing gate I would go for it!