I had the same problem ,at first. No sign of dead slugs.

However now ,when I put the pellets under the planks I see plenty of slugs and the proof of the pudding is that they can be within an inch or two of vulnerable plants like lettuce or baby spinach and there is ,surprisingly zero damage to the plants.

This for a week or two now.

So this is it. Look to see if there is any damage to the plants and if there is none you know that ,slugs visible or not your aim has been achieved and the plants have been protected.

But it is a lot of work . I have to go out each morning and check that there are still pellets beneath the planks.

Still, on the plus side ,by doing it this way my stock of pellets goes a lot further -and they are quite expensive compared to metaldehyde pellets .