How does the Laundromat tell whose shirt is whose? 

If you go to the laundry as infrequently as Gagler does, this can
be a vexing problem - he waits until he is on his next to last
dress shirt, then overwhelms the laundry guy with about 20 (or
more items) at one time. Considering a bunch of other guys, with
similar habits, also drop *their* laundry off on the same day
this could be a problem; you know the laundry guys just place
everyone's things together in one big washing machine!

Actually, to keep track of all of the shirts, etc., laundry
workers write on each shirt. Sometimes you can see it, sometimes
you can't. Another method is to write a series of letters or
numbers in indelible ink on the inside of a shirt collar;
customers might keep the same number while patronizing the same
Laundromat.

Another method used by cleaners is called phantom marking. Three
or four letters or numbers are written on the inside of the
collar using a special ink that can only be seen using
ultraviolet light. This number, which corresponds to the number
on your claim ticket is marked on the collar. While some
Laundromat's wash the shirts water in net bags, this is done
primarily to protect your garment rather than to separate it from
others.

After all of the laundry is dry, someone at the Laundromat passes
each shirt under an ultraviolet light and proceeds to sort it.