The only reliable way of characterizing between male and female goldfish is by looking for presence of white spots appearing on the fin and gills. However, this can only be spotted during mating season and thus expect to know their gender at least 10 months later when they are mature. (Find out more about Goldfish Reproduction)
Goldfish are heavy eaters and they are considered one of the most greedy aquarium pet fish. They will eat until they are full and if you decided to add more food, they will still continue eating until they become bloated. Usually this is also one of the reason leading to flipover condition. Therefore, do not get deceived by their enthusiastic behavior, which fools you to think that they are still hungry.
There are at least 20 known varieties ranging from fancy slow-moving delicate goldfish up to common hardy pond dwellers. They all originated from the same species meaning that if left together in the same fish tank, they will inter breed to produce off spring that resemble their parents. Some of the popular varieties are pearlscale, oranda, veiltail, shubunkin, tosakin and panda goldfish.
Goldfish can live up to 20 years provided that they are well cared for. The most important criteria to ensure that they survive up to that long period is by providing good water quality that remain consistent with their needs. They are coldwater fish (and not tropical species) and the suitable temperature should be within 18-22 deg Celcius and pH around 7.5. Watch out for your nitrite and nitrate levels as well and maintain it to the lowest level to ensure that your fish stays healthy. Therefore having pH test kit is very important.
Goldfish can grow up to very large size measuring almost 1 feet in length (including the fins) and the fish that you see sold in pet stores stuffed in aquarium tanks are actually juvenile fish. Because of this, never ever put them in fish bowls. My recommendation is to get a 55-gallon fish tank just to house a pair of goldfish.
Related topic: Finding Suitable Tank Size for Goldfish