Signs of knockout drops?

There is no substance called "K.-o.-Dropfen", but many different substances that are abused in this way. See: https://de.wikipedia.org/wiki/K.-o.-Tropfen

Unfortunately, the effects are often similar to those that may have excessive alcohol consumption alone. This is why it is difficult to recognize from the outside.

There are aids, such as bracelets, which can be used to test for some substances. But not everyone. The safest thing is still not to accept drinks from people who are unfamiliar and never leave anything unattended in clubs, etc.

If one thinks that one has been poisoned when one goes bad or else thinks that someone else is bad, in doubt it is always better to visit a doctor or call for help. Better “too early” than “too late”.

Maybe helpful: Safe through the night , https://www.kenn-dein-limit.de/alcohol consumption/jugend-und-alcohol/safe-through-die-nacht/