How to check if find command didn't find anything?

Count the number of lines output and store it in a variable, then test it:

lines=$(find ... | wc -l)
if [ $lines -eq 0 ]; then
...
fi

You want to use find command inside an if condition , you can try this one liner :

 [[ ! -z `find 'YOUR_DIR/' -name 'something'` ]] && echo "found" || echo "not found"

example of use :

 [prompt] $ mkdir -p Dir/dir1 Dir/dir2/ Dir/dir3                 
 [prompt] $ ls Dir/
 dir1  dir2  dir3
 [prompt] $ [[ ! -z `find 'Dir/' -name 'something'` ]] && echo "found" || echo "not found"
 not found
 [prompt] $ touch Dir/dir3/something
 [prompt] $ [[ ! -z `find 'Dir/' -name 'something'` ]] && echo "found" || echo "not found"
 found

Exit 0 is easy with find, exit >0 is harder because that usually only happens with an error. However we can make it happen:

if find -type f -exec false {} +
then
  echo 'nothing found'
else
  echo 'something found'
fi