It's hard to tell shubunkin females from shubunkin males. Shubunkins, or calico goldfish, are a type of fancy goldfish. A few telltale signs can help you tell them apart, though your best means of ensuring you have both males ad females may be to purchase in groups to increase your chances of getting both sexes.
Improving Your Odds
Goldfish are only somewhat sexually dimorphic, so the differences between males and females are not obvious. What differences there are won't manifest until fish are about a year old.
Shubunkins have a few minor physical differences. Males have slightly thicker fin rays on their pectoral or side fins. Additionally, females have more rounded bodies, particularly when viewed from above.
During Breeding Season
During breeding season -- spring if you keep your fish outdoors -- the differences between male and female shubunkins get more obvious. Males develop spawning tubercles, white bumps on their pectoral fins, heads and gill covers. Females' abdomens swell up with eggs, exaggerating their more rounded body shape.
Rolling the Dice
If you want male and female shubunkins, your best bet is to purchase a group of six fish. With a half-dozen fish, odds are good you'll get at least one male-female pair in the bunch. Most breeders employ this method.