How does one know if they're going through a mid-life crisis? And how is it different from the troubles people normally go through in life?

I'm not really sure.
The whole "get a flashy sports car and a 20 year old bimbo to spend my money" thing has never appealed to me.

There's a tricky, and very big, grey area. How are the surfing lessons going?

It’s a feeling that you’re missing out on things due to the passage of time, things you wanted or things You did when younger.

yes I think I've being going through a mid-life crisis for the last 20 years! :-)

If you hear Faith No More in the background

When the fear of growing old, loosing youth, or facing mortality drives decisions especially rash or non characteristic ones. Midlife crises are different from the troubles that people normally go thru in life because it usually marks a big change in the person and involves a crisis of identity at middle age.