With swift movements they can be just a blur, and unless they're sitting still, they can be very difficult to identify. So here are few things to watch for, so you will be able to differentiate the young from the adults. Just know that it doesn't take long for the young birds to behave like the adults, and don't think that you'll be able to tell them apart by size because the young hummingbirds will be very similar in size to the adults.
1. The most obvious is appearance, but even that can be difficult at times. Adult Males are typically smaller than the females. Their bill is a finer point, the wings are shorter, and when the tail is fanned out it has no white, but more sharply pointed. The most obvious is the RUBY throat, or appearing dark black without light reflection. The younger birds can appear the same size as the adult female, except in tail and bill length, but those grow to full length in days. It doesn't take long before spots start appearing on the throat of the young males. Those spots of black usually come in quite consistently, but the red feathers soon to follow, are very inconsistent, with oftentimes just one red bling at a time.
2. Watch their behavior - Young birds will be clumsy. They can oftentimes be seen slapping around in the flowers, making noises as their wings slap the flower petals. The young birds will use their feet a lot in feeding. They grip and grab anything to get good leverage on the flower they pursue. In this image, you can see shredded flower petals still pierced by his little talons. Young hummingbirds will spend a much longer time in the flowers before resting. Adult birds will spend much less time in the flowers, unless that's all that's available. They will usually taste and sample, but rest for more, and choose feeders far more. Adult hummingbirds will avoid awkward situations in the flowers. They'll select the ones that require the least amount of effort, and the ones most exposed.
3. Feeding from feeders - I've had so many people tell me that their hummingbirds don't like hummingbird feeders. First of all I must say that there isn't a single hummingbird on this planet that won't like a feeder, once it realizes what's in it. These occurrences are usually well after they've arrived, mated, and nested. It's always the young birds that go through the learning process of a feeder. They will spend the next week, after leaving the nest, sampling, experimenting, unable to resist their curiosity. They will literally spot every red object in sight, and often time fly right up to a feeder to examine the beautiful colors, and then turn away. Eventually they will learn from another young bird that's already solve them, or from the behavior of an adult male who spends the majority of his time defending that feeder. But they will learn, and when they do, life will never be the same. They will discover the value of that endless nectar, and not be able to resist the urge of addiction. They will soon fight like the rest, oftentimes showing bald patches and body scars, but knowing it is worth the effort.
August 9, 2019. N.E. of Edmonton, Alberta, Canada. Young male Ruby-throat hummingbird, feeding from a patch of Million Bells(Calibrachoa).