What is the way to know that the endpoint is down if there is an L4 / L7 balancer?

What is the way to know that the endpoint is down if there is an L4 / L7 balancer?

I'm writing an API-fuzzer and I want to detect if a sequence cause falling down of an endpoint of some service. Of course I can get 500 response code, but it's may be called from code of an endpoint. And there are any exact way to find out if a server is down?

I don’t know in advance what kind of balancing is on the service side. So I will be glad to know a universal solution, or some particular one.