Cows are very inquisitive animals; if they haven’t learned to fear humans they will come to you themselves.
Bulls may behave aggressively though, so I would not approach any standing alone in an open field (cows that are kept by themselves are either about to give birth* or bulls). If they’re in a herd they’re probably females, but young bulls may stay with their mother for a while, so always be cautious.
You might scare off cows if you’re unfamiliar and you approach them, I’d wait by the fence for them to come to me, if I were you. That way, if one will become wild, there’s a fence between you and them. This is what I do when I’m walking my dog and I encounter cows.
Just stick out your hand and let them do their thing. If this is the first time they see you they will shy away from any petting. Let them get used to you. They also love licking hands, I think because of the salt on our skin.
If they know you, they’ll come to your corner of the field immediately. ‘My’ cows come running when they see me at the end of the street. It took a while before we were at that point of our relationship though, have patience!
One thing the cows and I (and the dog) love to do is running. I’ll run along their field, and the whole herd will run with me, if I turn around they will as well. Even when I resume my walk they’ll follow me along the fence until they no longer can. They’re very playful! Their tongue is like sandpaper! What’s not to love?
Anyway, I’d still keep the fence between you and them. They’re big animals, they’re in a group and they can be very enthusiastic. Keep a bit of a barrier. Also the land they’re on is in many cases not public land, so best not to enter it. Cows are fantastic though.
*Edit: a commenter has pointed out that you shouldn’t go near cows with calves, they are likely to be protective. Quite right, always be safe people.