I have read the recent EULA for Windows 10 and don't see anything like that.
The License files are available on any install of Windows 10. They are in C:\Windows\Syswow64\Licenses\_Default. (the folders therein depend on the edition of Windows 10). The Windows 7/8/8.1 licenses are available similarly (license.rtf).
For Win7/8/8.1, the license effectively gives the user the ability to install and use the software on a single device at any given time. (Section 2a)
A Windows 10 upgrade uses that original license key. Therefore a Windows 10 install upgraded from Windows 7, 8, or 8.1, is using that license under the terms of the original agreement and installing the original version of Windows using that same license is in violation of the EULA by using it twice. This is in contrast to a retail installation which does have the option to upgrade but does not make use of the original license key.
But if that means I can not dual boot with windows 7 and the window 10 begot from my windows 7
It doesn't mean you can't. It means you aren't supposed to.
then why has MS given me some kind of warring that my key is not valid on at least one of the two?
It is not Microsoft responsibility to ensure you adhere to the terms of the EULA. Their current measures are intended to curb rampant piracy like somebody using the same key on 20 different systems. Right now you can do it with about 3 PCs before it stops activating the same key on different systems, for example; doesn't mean it's fine under the EULA to install it on 3 systems.
How can I find out if I am in violation of the present EULA for Windows 10?
You would be violating both the Win10 license as well as the license of the original OS.
FWIW, I don't care either way, but you basically said it was above-board on the basis that you were able to do it, which doesn't logically follow. That's the same logic that got me banned from the petting zoo.