How long ago was this? If it was high cocoa content, don't keep an eye but rush to the vet.I stupidly left the chocolate on a low table so when I got back Dennis had eaten them. I know dogs aren’t supposed to have chocolate, it wasn’t much but I gave him a raw egg to line his stomach and will keep an eye on him.
If you see symptoms, you might well be too late so keeping an eye won't help, it's not something which can always be repaired by the vet once the chocolate is in the system.
Happened here twice, both times I saw it happening so was at the vets within 30 minutes to make them puke the chocolate back up, most expensive chocolate I've ever had, but the dogs are fine.