Where do I see which affiliates have Custom Conditions?

In your advertiser account, go to Affiliates → Approved section. There you will see a list of all your accepted affiliates. On this listing page, under the Status tag (Approved), each affiliate with custom conditions is marked with a tag named Custom Conditions.

If you want to see which affiliates approved in your program already have custom conditions, next to the Affiliate Username, you will see a column named Custom Conditions. Thus, you can filter affiliates by that column, and the first ones at the top of the list will be those you have already granted custom conditions to.