how can I check availability of icloud email address

Try to create one. If it's taken you will get a message saying so. Of course only try this with an address you really want since if you can create it you won't be able to delete it if it's the main account address.

In theory you can check email addresses by using 'Finger' but it's not in the least reliable and doesn't seem to work with iCloud at all (and many ISPs simply refuse it). 'Ping' will return a positive whether the address exists or not (it checks the domain only).