ionic info
This will give you the ionic version,node, npm and os.
If you need only ionic version use ionic -v.
If your project's development ionic version and your global versions are different then check them by using the below commands.
To check the globally installed ionic version ionic -g and to check the project's ionic version use ionic -g.
To check the project's ionic version use ionic -v in your project path or else ionic info to get the details of ionic and its dependencies.