How to check overextension of my subject nations?

Playing as Great Britain, I managed to form the Thirteen Colonies in North America relatively early. With my permanent casus belli against primitives and superior military technology, I quickly subjugated several native tribes. Their provinces became a part of the Thirteen Colonies, and I was pleased to watch my subject grow and prosper.

...A few months later I noticed large rebel stacks in all of the recently conquered provinces, which eventually broke the nation, freed the tribes, and generally undid all my hard work. Turns out, I'd conquered too much, too quickly and the Thirteen Colonies, with over 100% overextension, had triggered some rather nasty events.

In future, it would be useful to know the overextension of my subjects ahead of time, so I know whether I should be going to war, or giving them some time to core their newfound holdings.

Is there a way to check the overextension of my subject nations?

Click on a vassal's province and hover your mouse over the unrest box. You can see the amount of unrest due to overextension, a quick calculation reveal the amount of overextension (5% of unrest = 100 % overextension). See here for more details.

Unfortunately there is no good way to actually check for the overextension of another nation (even your vassals/colonials). If you don't want to cheat, then your only option is to manually check every single province of said nation for non-cores. The amount of overextension is equal to 4x base Tax of all the non-cores (so if you have a non-core-province with 2 base tax and one with 4 that adds up to 24% overextension).

That being said there is always the cheaty option of loading your savegame as the nation in question (yes you can do that, just select your savegame and click on a nation on the map to continue playing as that nation) to check the overextension - or anything else you might like to know about that nation.