You can ask them to get a copy of their own credit file and show this to you or you can ask for a few months bank statements to see that they run their own finances well.
get the name of his bank & branch. Write to them and put in an authority from him (I authorise my bank to tell XXX about me and debit the fee to my account). Ask for the specific figures - can he afford �400 per calendar month by way of rental. The bank will reply back to you eg 'good for your figures and purpose' or something like that. The standard charge for a status enquiry is �8.50 which the account holder generally pays or you can put a cheque in payable to the bank for it.