To determine sex, you are going to have to look at your hamster's hind end and underside. Your best bet is to wait until the hamster is alert and relaxed to examine it.  Grabbing a sleeping or stressed hamster is a good way to get bitten. Try giving your hamster a treat and some attention. Hamsters don’t like being turned upside down (which is exactly what you’re going to do) so petting your hamster and making him or her feel at ease will help this process go smoothly. This will give the hamster a soft place to land if it squirms away from you. It will also keep your hamster contained at the same time, making sure that it doesn't fall and then run away. To safely restrain the pet, approach the hamster calmly, and do not startle it.  Place the palm of one hand over the hamster with your thumb and forefinger resting right above the shoulder blades. Gently grasp the loose skin just above the shoulders between your thumb and forefinger to control the head and keep the hamster from biting you.  Do not pull on the skin or attempt to lift the animal by the scruff alone.  Do not grip the scruff too tightly, either, as this can pull back the eyelids and causes the eyeballs to pop out of the sockets. Use the rest of the hand to support the animal's body. Using your free hand to support the weight of the hamster, lift the hamster and turn it over so that its body is resting in the palm of the hand you are using to scuff it. Its entire body should be supported at this point.

TL;DR in Indonesian:
Tunggulah waktu yang tepat untuk memeriksa hamster. Usahakan menggunakan mangkuk atau kotak besar yang dilapisi handuk. Ambil tengkuk hamster. Angkat dan jungkirkan hamster.