How to tell if you need a new chain??

[Two]

Well the scientist in me always used two reasons, 1) a clicking link, or 2) out of adjustment with tendancy to slip

30K is great, and always make sure you replace as a set ( sprockets and chain ). Obviously that doesn't apply to those swapping out rears all the time for racing, etc.

Front and rear sprocket or just the the rear?

[macon454]

Front and rear sprocket or just the the rear?

Always replace both sprockets and chain at same time

[dadofthree]

Front and rear sprocket or just the the rear?

Both, always both.