I live in Kentucky. I plan on having a social get-togeather soon, am I liable for underaged drinking if I have signs posted of no underaged drinking?

Yes. What part of underage drinking don't you understand? The posters that say "Parents who host lose the most" means just that. If there is any underaged drinking on your premises regardless of posted signs or not you are still committing a crime and can be punished for supplying liquor to a minor or allowing a minor to consume alcohol on your premises.