Do you really want to know? As mentioned it's a lost and arcane art and they have been getting away from it.
With a few chickens I don't think I'd would ever bother to try. Essentually it involves the differences in the sexual reproduction organs and your ability to detect them in young chicks so you can grade and ship pullets only to those interested in egg laying/production versus chicken production.
I won't bother you with all the gory details but it essentually involves the use of a sterile glass rod or pipet and inserting it into the chicks cloaca and fealing to see if it diverges into the ova duct. Female chickens start out as infants with two ovaducts but one, the left one atrophies soon after hatching.
Cockerals of course don't have an ovaduct and the rod dead ends but you have to be real careful to not cause damage when testing or to not transfer bacteria etc. The rods are cleaned and washed in a sterilizer (alcohol etc when in use and then autclaved at night)
The glass pipettes must be kept sterile and clean. To be honest I've never cared enough to learn how but they tell me the real good people could do several hundred chicks and hour. Lenore, the eldest sister that ended up with the biology degrees was very good at it and made really decent money when we were teens doing it for Dunlap Hatchery in Eagle, ID, Nice , clean , airconditioned work space with acomfortable lab table and chair while the rest of us dummies bucked hay, irrigated, chopped weed setc.
Now, at least for pullets for egg production they have developed breeds, Black Stars and Red Stars come to mind that the chicks are sexually dimorphic at hatching, different colores for pullets versus cockerels so now it's easy to tell for many of the commercial breeds.
The advice if you only want a few, to buy 2 times as many as you want if you want a specific sexaul grouping, and don't want too many, is still pretty vallid.
This year I bought Black Star pullets for $2.73 each and Aracunas and Marran pullets for $3.55 each. That was because the Aracunas and Merrans still have to be sexed if you want straight pullets, so you pay for the test.
Bottom line is there is no easy way, that you are going to learn in twenty minutes so why bother. Either buy breeds that are sexually dimorphic at the time of hatching or simply buy extra if it's not a huge number.
Nearly all commercial operations can/do sex chicks so you can get what you want, you just pay a bit more if it actually requires testing.
Per: "The Chicken Health Handbook" by Gail Damerow, Storey Book Press, Pownal, Vermont.
rwt
