Hi Rosario, good questions, this is tricky.
(1) Your client should ask for bank details and run a credit check.
(2) Also check with the Better Business Bureau (if your client is in the United States).
(3) Ask for customer referrals.
Here is a website with resources for a few countries: http://www.wholesaleforum.com/discuss/wholesale-discussion-24/how-verify-your-wholesale-suppliers-best-techniques-78/
Hope that helps,
-Kwaku with Now In Store