I am not aware of any documented api call that can be made specifically to validate keys. Here is a suggestion you might try:
Require your partners to provide a valid credit card and inform them that in order to validate their Stripe keys, you will be making a $0.50 charge to their card that will be immediately refunded.
As part of your form validation, when both keys are given, submit a hidden form that contains all the data necessary to create a card token. You should be able to examine the response in your create card token response handler and determine if the publishable key is valid.
If you get a successful response back from the stripe server containing a card token, turn right around and submit a test charge for $0.50 (the minimum charge amount).
Make sure you're properly catching all the stripe exceptions. I believe with an invalid secret key, you should catch a Stripe_InvalidRequestError. If an exception is thrown you can report to the user.
If no errors are thrown, the charge will be made. Since you don't want to charge your partners, you'll want to capture the charge id from the stripe response and immediately refund the charge.