It is not a fool proof method of sexing Welsummers but in most cases, some of the BYCers had better luck than I have.
The solid V's and eyeliner marks are females and the light or fuzzy V's are males.
Holding them upside down is an old wife's tales. Just about all chickens will flay or struggle when you pick them up.
Are you having those chicks from a hatchery or private breeders? If you are ordering from a hatchery, be prepare for either beautiful eggs or simple brown eggs that do not resemble anything close to terra cotta colors.