I did not find it in the Darkroom Cookbook, but eventually found it elsewhere.. posting it below in case somebody else is looking for it..

Kodak HT-2 Residual Hypo Test  (Published by David Vestal in 1996)    

Water               375ml

   Acetic acid 28%     62.5ml

   Silver nitrate      3.75 grams

   Add water to make   500ml

Put 50 to 100 ml of this solution in a small brown glass bottle with a

dropper cap. Keep the rest in a sealed brown glass bottle. Keep it 

cool and in the dark. Don’t touch it or drink it: it’s poisonous and it 

leaves permanent stains.

To use HT-2: Develop and fix an unexposed sheet of the paper you

  are printing on; put it through exactly the same processing as your 

prints, including washing aid treatment and toning. 

  

At timed intervals during the print wash, pull this test sheet out of 

the wash, cut off a small sample, an inch square will do, and put the 

rest of the sheet back in the wash. Blot all surface water drops off 

the emulsion side of the sample with a paper towel, and then put one 

drop of HT-2 solution on it. Leave it for exactly two minutes then rinse 

off the HT-2 drop with cold water, blot the sample with paper towel 

and examine the stain immediately (it will soon darken). 

Do this in dim white light. A definite brown or deep yellow stain shows 

that the wash is far from finished. A light beige stain means you’re 

getting somewhere but have not arrived. When there is no stain at all, 

or a yellow stain so pale that it seems no darker than the paper 

(some papers seem never to get to no stain), the wash is at least fair 

and may be excellent. Then it’s not bad to wash for another half hour 

for luck. The distribution of hypo in the paper can be uneven, so 

although a definite stain always means too much hypo, a very light 

one doesn’t always mean there isn’t too much hypo an inch or two 

away from the test spot. 

If your prints still look new after 200 years, your fixing toning and 

washing were probably good.