I don't fish for trout very often, but feel the answer is the same no matter what species you may be targeting. Know the available forage & the sizes, then select based on what is there. Even when there are no hatches, there are still usually insects in various forms, such as nymphs, or possibly terrestrials depending on the time of year & where you are. There may also be various baitfishes & juveniles of the prevalent fishes in the area. There could also be such prey as crayfish, or many other aquatic animals. Just because there are no hatches doesn't mean the fish you target don't eat.
For me, because I primarily fish for bass, I'll usually start fishing with some type of streamer pattern. Bass eat smaller fishes, and they're not usually particular about what those smaller fishes may be. In some places I've fished for a long time, I know what patterns have worked the best so experience helps with fly selection, but if I was starting fresh in waters I've never fished before, I can still be reasonably sure of my selection. Bass are bass no matter where they are found.
I take the same approach when I do get to fish for trout. Most often that's in the summer, so when there are no hatches, I'll start with terrestrials, which is a good bet in my area. In some locals, specific fly patterns may work best, but until you know what those may be, it's not usually difficult to figure out what might be the available forage, simply by taking a few minutes to look around & be observant. A bit of research before hand doesn't hurt either. In this day & age, the information that is available is staggering. If you don't take advantage of it, you can only blame yourself. State Natural Resources websites are a great place to get such information.
Of course, if hoppers are on the menu & you have none, then that's something you just learned the hard way. Keep it in mind for the next trip, even keep notes if you so desire & be better prepared.