I know there are many tools to test for bandwidth and latency, but they all rely on having at least one host from which you can run those tests.

I wonder whether there's an online source or some other way to guestimate the latency or speed between two countries (in general).

For example, would a customer in Japan get lower latency if the server is located in Singapore or Australia? Is a user in India likely to get higher download speed from a server in the UK or in the US?

Are there any online resources or some clever ways to answer those questions with a reasonable degree of accuracy?

[UPDATE]: Thanks for the great suggestions from Raffael Luthiger. I didn't know about those looking glass servers. The submarine cable maps were also really cool to discover (Thanks to Jesper Mortensen). Also seems really wise if I could ask those network professional in the area for their experience, but obviously I don't have access to those. At least some of them are on SF :)

However, I'm still a little unsure how to combine those resources to give me some measurements. This is the information I have: Two countries (A,B). I do have IP addresses of customers in country A (I can obtain those from the web server log files for example). Presumably I can find some looking glass servers in country B and run a trace to those IPs. What's the best measurements to use? Are there any scripts that help automate at least some of this process?