In PowerShell you can use the following to get the target runtime:
$path = "C:\Some.dll"
[Reflection.Assembly]::ReflectionOnlyLoadFrom($path).ImageRuntimeVersion
I adapted this to PowerShell from Ben Griswold's answer.
If you want to know the target framework version specified in Visual Studio, use:
$path = "C:\Some.dll"
[Reflection.Assembly]::ReflectionOnlyLoadFrom($path).CustomAttributes |
Where-Object {$_.AttributeType.Name -eq "TargetFrameworkAttribute" } |
Select-Object -ExpandProperty ConstructorArguments |
Select-Object -ExpandProperty value
You should get something like
.NETFramework,Version=v4.5.2