Taller than average height is a dead giveaway that you should investigate them as a possible prairie candidate. Their healthy looking appearance, which signals a lack of predation, is still yet another clue. The prairie plants will at first be the unusual ones, so identifying them really is not that hard. It's waiting for them to appear that is the tough part. When your plants, both grasses and forbs, mature enough to begin producing flowers, you can use the flower color to help identify them.
Most plant identification guides group plants first by the color of the flower they produce.
The trick here is that some of the colors are subtle and it may be hard to determine if the flower is one particular color or not. In these cases you will have to choose one of the colors and search the guides to eliminate one or the other color choices. Then they subdivide those plants by the actual number of the flower petals or shape of their seed heads. If you are using an illustrated guide, after you get to the point of determining all this, there is usually a picture of the plant you are trying to identify. There are many books and guides which help you find the identification of plant species.