Not finding bash commands

I was wonder with someone can help me:

if [ -z $1 ]; then                                                                                                                                                                                                                                                             
  user=$(whoami)                                                                                                                                                                                                                                                         
else                                                                                                                                                                                                                                                                           
  if [ ! -d "/home/$1" ]; then                                                                                                                                                                                                                                           
    echo "Requested $1 user home directory doesn't exist."                                                                                                                                                                                                         
    exit 1                                                                                                                                                                                                                                                         
  fi                                                                                                                                                                                                                                                                     
  user=$1                                                                                                                                                                                                                                                                
fi 

I was studying some bash commands when I saw two commands: -z and -d. I know what they do (first check for blank variable and the second check for existence directory). My question is how I can find descriptions about these commands (i.g man page -d/-z). They can be only used with if-else statement?