How to check if sites are lying?

Question

Parsing

Good day, this is the essence of the question:
There are n-th number of sites (list), and you need to check if they lay down, maybe there is a ready-made solution, or where to dig at all? So that the finished result with all the underlying sites is recorded somewhere, and as a result, this problem can be fixed.

Answer 1

https://uptimerobot.com/

Answer 2

https://ping-admin.ru/

Answer 3

https://www.host-tracker.com/en/