As well as being the developer and PR agent for jetwick, sadly I'm also the admin . All in one... at once. Here is a minor snippet to get an alert email if your Solr index is either not available or contains too few entries. You'll  get a resolved mail if all is fine again.

cd /path/
FILE=bla.log
EMAILS="your@email.here"
SUBJECT="OK: jetwick"
STATUS=OK

CNT=`wget --http-user=user --http-password=password  -T 10 -q "http://your-host.com/solr/select?q=&rows=1&wt=json" -O - | tr ',' '\n' |grep numFound|tr ':' ' '|awk '{print $3}'`
if [ "x$CNT" == x ] || [ "$CNT" -lt 500000 ]; then
  SUBJECT="CRITICAL: check http://your-host.com/solr"
  STATUS=CRITICAL
fi

PREV_STAT=`cat .status`

if [ "$STATUS" == "CRITICAL" ]; then
if [ "$PREV_STAT" == "OK" ]; then
cat $FILE | mail $EMAILS -a $FILE -s "$SUBJECT. doc count was only $CNT"
fi
echo CRITICAL > .status
else
if [ "$PREV_STAT" == "CRITICAL" ]; then
cat $FILE | mail $EMAILS -a $FILE -s "SOLVED: http://your-host.com/solr"
fi
echo OK > .status
fi

echo $STATUS > .status

add via this via crontab -e

*/2 * * * * /path/check-health.sh

If you look at the code there is one mini hack which is necessary if the solr index is down and the CNT is empty:

"x$CNT" == x

From http://karussell.wordpress.com/2010/11/25/poor-men-monitoring-for-solr/