I’ve found the easiest way to identify a juvenile is to look at it’s feet. From the point that it fledges until it is several months old the juveniles feet are much duller than the bright orange colour of the adult birds. The younger the bird the more mottled the appearance of the feet.
Another give away to look out for is the pale tip on the beak of a young bird. It’s been suggested that the pale tip is adapted for life in the burrow so the adult can recognise where the chicks beaks are situated in the poor light inside the burrow. Once they emerge, the pale tip starts to fade, but it’s still clearly visible in birds a couple of months old.
The final things to consider are more general. Youngsters often hang around together and are less shy of people than adults, so if you see 2 or 3 birds in reasonably close proximity to each other, they will probably be juveniles. Their plumage is just a bit duller overall than the adult birds and they appear a bit more compact in appearance than the adults.
My top tip for identifying a juvenile though is to look at the feet!