LAUGH MACHINE OR REAL LAUGHTER?

Mostly it's real laughter but the people are also prompted and told when to laugh, when to cheer etc etc. It's hard to tell when it's a "machine laughter" but I'm sure those shows are also edited, laughter wise. Pitches have to be adjusted, shortened, and all that stuff. As long as you find it the jokes funny in the end, that's all that matters :)

It usually depends on the show, but I think it would be easy to tell the difference. If it is a prime time show it is usually using a laugh track. That wikipedia article about laugh tracks says "How I met Your Mother" uses it too. Personally, I enjoy the show more when I hear the announcer say the show has been recorded in front of a live studio audience!