This can be achieved by using following steps
1. Login to your sandbox.
2. Go to Developer Console.
3. Go to Debug menu and click on Open Execute Anonymous Window
4. Paste the following code in that window.
ApexClass apexClass = [
SELECT ApiVersion
FROM ApexClass
WHERE Name = 'YourClass Name'
];
System.debug(apexClass.ApiVersion);
5.Run this code.
6. This will print the API version of your Apex class.
