There're a few ways you can verify the validity of postback URL from your affiliates. Below is the method you can adopt to test on Offerslook.
Go to Offers -> Conversion Pixel menu.
Click the Test icon on the right of the Affiliate Postback to be tested.
Retrieve the test link from your affiliate, and insert it into Affiliate Test Link field.
Click Test and then click Conversion; the testing message will appear underneath, and tells you whether the test has been successful.
That's it. You're done. If the test messages tells you conversion has been successful, you can go to the Test Conversion Report to view the test statistics!