If you have a picture of an airtime hill (and nothing more) it is very tricky to predict if it will generate airtime or not. You can obviously take a rough guess but to actually tell if it creates airtime or not you have to know how fast the train will travel through the hill. So if you have a video it is much easier, also, if in the picture you can see that there is a big difference in terms of height between the airtime hill and the previous element you can predict that there will be plenty of speed through the hill. If you had an airtime hill which had a beyond vertical entry or exit (which I've never seen) you could say without a doubt that there would be airtime.
Usually when I see a POV I can tell if it will generate airtime or not although it might, sometimes, be hard to tell if there is just near weightlessness or light negative Gs. (when there is ejector air it is naturally easy to tell)