Cattle will ride if they are crowded up. But if they are truly standing, that is the one on the bottom doesn't move for it. Then it is a pretty good sign that they are in heat. If you're not sure that they are actually bred, then getting them checked is a good idea, you may have a bull problem.
Now, having said that, we have observed several of our cows (mature cows) standing this winter. They were pregchecked in the fall and are all supposed to be bred. They look pregnant, and I am pretty sure that they are going to calve. I've never seen the same cows 'in heat' again. I've seen the same thing in the past, as well, where pregnant cows were standing. And I know that they were pregnant, cause they calved in the spring.
So if I were pretty sure that the animal/s in question are pregnant, and I only saw them standing 1 time I wouldn't get too worried. But if I were seeing the same animal/s riding regularly I would be suspicious and have them checked out as well as the bull they were supposed to be bred to.