That really would be the only way to know if the drug is working or not. Again, I'm not all that familiar with internal parasites, but I'm *assuming* (assumption only) that more serious cases might warrant another round of treatment if the parasites were not fully wiped out the first time. It would certainly be something to ask your vet.
Also, I'm almost out of fenbendazole. I've got 5 tablets of panacur (150 mg each). So, I'm worried about not being able to find any in this lockdown, if I'm out of it. My question is, can I create a solution in water of these tablets and then administer from it as and when required? Will being dissolved in water for long deteriorate the impact of the drug?
You could quarter the tabs and crush a quarter, mix with, say, 1cc of water, give 0.5cc per dose, refrigerate the balance for the next dose. I think this would put you near the dose you were aiming for.