As a Skimlinks publisher you see a specific set of merchants based on the domain you select in the Publisher hub.
The easiest way to know if a merchant you are interested in working with is available is by checking the Merchant search.
The surest way is by checking the your clicks in the performance reporting. If you can see clicks from your account to the merchant then you are able to earn affiliate commission from that program and they are live.
If you are seeing clicks and not sales and want to understand why that may be the case please see the article Why am I seeing clicks, but no sales?