ng version
You installed angular cli globally (-g in the command). This means that you can type ng version into your command prompt. It may be more precise to do this when your command prompt is not within a npm controlled directory (you should type this in within directory you typed ng new myapp).
A note to those who got here from Google: ng version will let you know which (coarse) version of Angular is referenced by the current directory.
e.g. This directory appears to have angular 4.x (~4.3.0) installed.
@angular/cli: 1.2.1
node: 8.11.1
os: win32 x64
@angular/common: 4.3.0
@angular/compiler: 4.3.0
@angular/core: 4.3.0
@angular/forms: 4.3.0
@angular/http: 4.3.0
@angular/platform-browser: 4.3.0
@angular/platform-browser-dynamic: 4.3.0
@angular/router: 4.3.0
@angular/cli: 1.2.1
@angular/compiler-cli: 4.3.0
If you are not within a directory which has a packages.config, then you will get Angular: ....