I'm using powerShell, and I want to use lower-case() in a XPath.Something like that
//Machines/Machine[lower-case(@FQDN)=lower-case('machine2.mydomain.com')]
The documentation indicate that I need XPath 2.0, how can I check the version of XPath installed on my server?
As of 2012, Microsoft was officially not supporting XPath 2.0 in the .NET Framework (which is what you'll be using from PowerShell, unless you pull in a 3rd-party library) and as recently as last year it was still not happening. Given that they said a decade ago that they weren't going to implement it, and it hasn't happened yet - don't count on it ever happening.
See XPath and XSLT 2.0 for .NET? for alternatives.