How to notice column changes in trigger

How can I detect whether a column was changed by an update command
inside a trigger?

create table test(a int, b int, c int, primary key(a))

b and c should be updated inside an update trigger if not modified by
the statement itself

1) update test set a=0 -> trigger does its work
2) update test set a=0, b=1, c=2 -> trigger does nothing
3) update test set a=0, b=b, c=c -> trigger does nothing, but content of
a and b dont change either although touched

What I'm looking for is something like
IF NOT COLUMN_TOUCHED(b) THEN ...
For MSSQL, this would be coded as IF NOT UPDATE(b) ..

Any hints?

Andreas