This article is from the Sourdough Starters FAQ, by Brian Dixon briandixon at hotmail.com.
The starter has a nice, smooth consistency. It is
filled with tiny bubbles throughout the starter above and below the
surface. It typically has a layer of frothy foam covering most of
the proofed starter. The froth typically appears as early as 8 hours
into the proofing period and lasts until about 18 hours of proofing.
Stirring the starter obviously releases a lot of gas (smells good).
Examining the starter clinging to a spoon shows that the starter is
chock-full of little bubbles. The starter quite often appears puffed
up when the proof is done and drops down to a lower level upon
stirring. As a final check, starter that you expect to be classified
as healthy, should be able to raise plain white bread dough in 2 1/2
hours or less. It's probably not worth experimenting with raising
dough until all of the above characteristics of healthy starter are
present. Congratulations! If your starter is like this, you can
pronounce it fresh, vibrant, and healthy! It's ready for bread
recipes and will now be much more resilient to abuse and mishandling
and should be very reliable now. Skip the rest of the instructions
for "new starters".
What should you do if you have "dead", "flat", or "barely living"
starter? Begin the process of transforming it to a fresh, healthy
starter. I personally do not believe in throwing away "dead"
starter, since it typically can be revived from the few yeast and
lacto-bacilli that probably still exist. If restoring dead starter
takes longer than a week to see bubbles appearing in it (flat, barely
living or otherwise) then you've probably started a new starter from
local microorganisms. If so, and your starter was a special strain,
you'll probably want some of the original starter to start over with
rather than expecting this revived version to be the same as that
special starter. Remember that you have probably not really killed
your starter unless you subjected it to high temperatures for long
enough to thoroughly heat the starter above about 100F or so.
Here's the "get it going" reiterative process I referred to:
 
Continue to: