Indexation is the right first check. There is a second one almost nobody runs, and I say this as someone who sells links, so weigh it accordingly.
Count the outbound links on the host page. Count them again in three months.
The page carrying your link plus two others when you bought it can be carrying fifteen by spring, and nothing tells you. Your link never moved. It was not removed, it did not fall out of the index, it still passes every check in this thread. It is simply worth a fraction of what it was, because the operator kept selling slots on the page that was already working.
That is the quiet failure, and it looks exactly like success from the outside. So log the outbound count on the day you buy, then re-crawl quarterly. A host page whose outbound links keep climbing is being farmed, and that trend only runs one way.