You're at a party, and you want to know if it's ok to flirt with that hot person who came with the hetero guy. Are they just friends, or lovers?
Easy: if it's a man, they're friends. You may occasionally be mistaken, but 99 times out of 100, you can distinguish between friends and lovers by gender.
If you're not sure, check their level of physical intimacy. Heterosexuals rarely sit with their hands on their friend's knee, or make out with them, or go down on them.
If you're still not sure, find out if they live together. Heterosexuals over age 25 usually shy away from living with friends, thinking of it as "juvenile": grown-ups should only live with lovers.
With gay men, it's not so easy.
1. Friends and lovers are typically both men.
2. Friends have no qualms about physical intimacy. They are perfectly willing to make out with you, or to go down on you. But sometimes lovers are sexually incompatible, and rarely do anything.
3. Friends often live together, and lovers sometimes do not.
4. Guys often have very close, inseparable friends.
But you have to be able to distinguish. It's extremely rude to invite one lover but not the other to a party, to give a Christmas present to one but not the other, or even to hold a conversation with one without asking "How's your boyfriend?"
It's even more rude to cruise a guy's lover.
Mike and Calvin were two guys who worked out at Barney's gym: both in their 30s, buffed but not bodybuilders, with smooth chests and big biceps. Mike was taller, pale/Anglo, and a little more buffed. Calvin was shorter, dark/Hispanic, with longer hair, and a bigger penis (I saw them both in the shower).
I was interested in Calvin. But I couldn't cruise him openly, until I knew whether they were friends or lovers.
The full story, with nude photos and sexual situations, is on Tales of West Hollywood.