Unless someone tells you they are gay, or you have some other concrete reason to suspect it, the only way to know is to ask them. Be sure to keep in mind that it's not your place to tell people who they are, and to be careful with how you word your questions, as putting a label on someone can be very insulting.
Some ways to tell if someone is gay include:
Seeing them with friends of the same sex, or bringing dates when they come over to your house. Having a high-pitched voice, which is common for many homosexuals. Wearing makeup, clothes, or jewelry that is more feminine than the person usually wears.
If you think that your brother is gay, try not to treat him differently or make a big deal about him bringing home a date or being in the fraternity/sorority with a guy/girl. You should hold him to the same standards as you would any other brother/sister.
Family secrets take a toll on everyone involved. You and your brother may benefit from talking to a counselor or therapist, although this should be a personal decision that he makes at his own pace.
In the meantime, the best thing that you can do is to educate your parents about the damage that secrecy does. Hopefully, that will encourage them to be more open minded in the future. Ultimately, the more your parents learn about the effects of homosexuality on their own lives and in their families, the more they might see that there is no danger in letting go of the lies they have been telling.