Weak charging?

[Steven Bixby]

So, my GPZ has been mostly unused recently and I brought it back to
life last week while my ST13 is being serviced (by me, over several
days).

One issue I know I need to get after is to pull the carbs and look
them over; I am fairly sure that there is some idle-circut clogging
occurring as it hesitates very badly off-idle.  But that's besides the
current point.

The other issue that I'm concerned about is that I don't think the
charging system is getting much done - I suspect the battery isn't
getting charged.  The bike will run once I get it started but it
always seems to be weak on the restart and I've put the charger on to
take care of it a time or three.   I've bumped started it another time
or three in the past few days too.

So - how does one diagnose this?  I've never tried to check a charging
system before.   I have some measure of electrical know-how and can
drive a DVOM fairly well.  I suspect I need to get a shunt in-line so
I can check the current flow into/out of the battery, but if there's
some inexpensive diagnostic tools out there already, that'd be great.

If the alternator is headed south - is it difficult to replace?  Are
there other points of interest besides the alternator that would cause
poor charging?

BTW, the battery is about 6 months old, so I don't *think* it's the
problem spot.  Once I put it on the charger for 10 minutes or so it
usually spins the starter with great gusto, and there's enough voltage
to drive the spark.