Most noticeable would be death. lol You can't really watch for clamped fins or no eating.
If you want to add shrimp in with your betta I, and many members here, would recommend amano shrimp bc they're large enough to defend themselves. I lost one fully grown adult RCS to my small-mouthed Bowser (I picked out my 2 largest rcs to help control algae but it didn't work out. They were so red and pretty too) Some people manage to keep ghost shrimp but where ghosts were overbred for food and other experimental things such as disposable tank cycling tools they are susceptible to less than are rcs and amanos (who were selectively bred to be versatile for a wide range of home tank environments)
As for snails. I kept ponds and ramshorns before, they're pretty quick to die in the wrong conditions and many people see them as pests anyway. I have no experience with mystery or rabbit snails (people here on the forum do though) but Iknow they poop a lot! And they grow really big. I keep nerites (well, one for now) and she has a very low bioload and eats all the algae. Nerites exclusively eat algae (I could NOT get her or my old nerite to eat algae wafers) so I would recommend nerites for a tank with algae problems. I haven't fed Milly in 8 months, she is very robust and has almost doubled in size! lol
In a q-tank, you wold simply watch for activity levels, how much they eat (and what they eat) and how they act after a water change and such.