Babies are easier because they leave the faintest stain on the diaper the first week or so.

Then it becomes more but not much scent, like a training period. And it's really only 4-6 months that it begins to get foul.

Another advantage for the baby side is a hack I can't believe more people don't do. Just notice when they make a very recognizable face or start grunting then hold them over the toilet.

False positives are no big deal (just a fun change of scenery for the baby), for false negatives you change the baby as normal.