First indicator is the comb. A pullet (female chicken) of this age won't have that large or red of a comb yet. Females don't get large red combs until they're ready to lay, around 5ish months. That's universal to pretty much any breed.
Second is unique to barred birds. Due to the sex-linked nature of the barring gene, males get two copies while females only get one. A black barred bird is genetically black, and the barring gives those white bars or stripes of color over the black base. Hens get only one copy of the gene, so they have smaller bars of white. That makes them look darker overall. Males get two copies of the gene, so they have twice as much white as females. This gives them a much lighter overall appearance.