AGE is the most important factor when looking for signs to determine if your bird is a pullet or cockerel. Do you have to wait for an egg to be laid or for crowing? Nope. Not if you know what to look for. Knowing their age is important and that's why I usually ask.

Young female under a year of age = pullet

Young male under a year of age = cockerel
Female over a year of age = hen

Combs and Wattles... looking for prominent growth and redness by 8 weeks of age in comparison to the other chicks from the same hatch and line is generally an accurate way to sex birds. A bright red comb this early is almost always a male trait. Please note that yes, pullets get bigger, red combs and wattles around sexual maturity. But rarely do you see this before 12 weeks of age. You may see combs pinking up, but not red. After 12-16 weeks this line gets a little more blurry as pullets start approaching laying age. I hear people say, "my hen has a big red comb too, so that isn't accurate!" You need to keep in mind that you cannot compare a mature hen to an 8 week old cockerel.

A note for peacombed breeds: the "3 rows is a male, 1 row is a female" saying is a myth. Females can have either. You should be looking at SIZE, PROMINENCE, and early REDNESS, not how many "rows of peas." The ideal female peacomb has 3 distinct rows, just ask a poultry judge.

Saddle Feathers... these generally start growing in around 2-3 months of age and are the MOST ACCURATE indicator to confirm your suspected cockerels. Sometimes you can see them earlier than 2-3 months if you look underneath the top feathers on the back, above the tail. A lot of times these hide underneath for quite a while and will not be evident from a quick glance or from a picture. If you check too early, it's not as accurate because nearly all chicks will get rounded feathers at first. The male sex feathers generally come in last, after the first chick molts. You're looking for pointed, thin (sometimes shiny or darker or differently colored) feathers on cockerels. Pullets will be growing in wide, rounded feathers as part of this stage in maturity. Yes, every once in a while, a late blooming cockerel will get saddle feathers later than this, but it's not as common. If you see these specifically shaped saddle and back feathers, there is no doubt you have a cockerel.

Neck Hackle Feathers... Pointed neck hackles usually don't start coming in until AFTER saddle feathers do, so looking for these, especially early, is not very accurate. Some varieties or mixed patterns make it more difficult to determine if hackle feathers are truly pointed or if they rather appear that way because of the color pattern.

Curving Tails... you can't judge the tail of a young cockerel in comparison to a mature cockerel or cock bird. Those curved sickle feathers can take a while to grow in, usually later than the other tail feathers. Sometimes folks mistake curved tail feathers on pullets for "sickle" feathers. Feather placement and shape is more important than curvature in young birds. Green sheen in the tail is also not a reliable way to sex young birds, as many color patterns require sheen in all black feathering on both sexes.

Wing sexing (day olds or 2 week olds) by wing shape, feather length, etc is NOT accurate for most birds. This trait has to be bred for. You need the right combo of genes for it to work. Yes, generally females feather in faster than males, but this is also not an accurate way to sex most chicks. Looking for early tail feathers or longer wings is not reliable at any age. See more info on the genetics of feathering rates here.

Vent sexing... takes a lot of experience and is not even 100% accurate for the professionals that look at thousands of chicks each day. I would not recommend trying this at home. You can severely injure chicks.

Sexual Dimorphism... Some color varieties can be sexed earlier than others. Sexually dimorphic varieties will have feathers grow in different in color. Example: Wheaten males feather in darker and females are a light tan. You can see the difference in the first few weeks of feather growth. Wildtype birds can also be sexed early based on early feather growth (dark male chest feathers, salmon chests in females). Wide, double barring is always a male color pattern. There are several color patterns that are sexually dimorphic and sexing this way is accurate for all breeds and crossbreeds that exhibit these specific pattern genetics. 

Leakage... If you see color "leakage" in the sex feather areas, this is generally a male trait. Sometimes this will be apparent even before pointed saddle and hackle feathers grow in, most specifically in the wing bow/shoulder area. If you see red/rust color or even gold or silver here, you likely have a cockerel. Even on many solid birds, these areas will have darker feathers. Example: a blue cockerel generally gets darker blue sex feathers and you can see this in the early plumage in the wing bow/shoulders. A red or buff bird can be darker here.

Autosexing breeds and sexlinks can almost always be accurately sexed at hatch using barring or other sexlinked traits. There are several ways to create sexlinks. If you have more questions about that, feel free to message me on Facebook. A white headspot at hatch only means the chick is male if it was bred that way.

I hope this helps you learn what to look for and give insight into what I am looking for when I'm asking to help based on pictures. If you have any questions, feel free to ask.

Edited to add:

There are some breeds that are considered "hen-feathered" which means males and females have the same feathering. Males do not get pointed saddle feathers. An example of this is Sebrights.

Most of these things still apply to SILKIES, however, it may be harder to tell as early due to their fibromelanism and silkied feathering. 

You can have chicks DNA tested for sex via several different labs, if that interests you.