How can I know on which drive is Windows installed?

There are a few ways, and I'll mention the quickest ways that come into my mind.

1. Open Run dialog (Winkey+R) and type the below command and Enter, it'll open your current Windows installation directory

   %windir%
   

2. Open Task manager and select a system process (something like svchost.exe or winlogon.exe) in Details/Processes tab. Right click on that and you can see Open File Location, which will also open your windows directory.

Some other ways

• Open the run dialog with Win+R, type \ then Enter. That'll show your Windows drive. (Tip: running . will show your home folder and .. will show your users folder)
• Open start menu, type "system info" and open System Information. You'll see the Windows Directory under System Summary
• Open start menu, type "disk management" or press Win+R > diskmgmt.msc > Enter. That'll list all the drives in your PC. The system drive will be listed with the Boot flag, most likely along with "Crash Dump" and "Page File"

• Simply press Win+R and run cmd. The path to cmd.exe is often shown on the title bar by default. On some systems it simply shows "Command Prompt" but in that case you can look at the prompt string which often points to your user folder in the system drive

  

  This is quick but not reliable either, so while you're at the commmand prompt just run the commands in other answers or you can run echo %systemroot% or set sys

  C:\>set sys
  SystemDrive=C:
  SystemRoot=C:\WINDOWS
  

• Press Win+Pause or right click My Computer > Properties > System Protection. Your system drive will be listed with the word "System"

  

That said, nowadays Windows often shows its system drive always as C: regardless of its system volume position in a drive, so a drive label or device name is more useful than a drive letter

The drive letters like C: are only assigned during Runtime of a Windows installation, so they will usually be both C: for both your installations.

Open an elevated cmd.exe or Powershell.exe (as Administrator), then type:

diskpart

after a moment, type:

list volume

You get something like this:

 Volume ###  Ltr  Label        Fs     Type        Size     Status     Info
 ----------  ---  -----------  -----  ----------  -------  ---------  --------
 Volume 1         WinBOOT      FAT32  Partition    350 MB  Healthy    System
 Volume 2     D   Win1         NTFS   Partition     31 GB  Healthy
 Volume 3     C   Win2         NTFS   Partition     33 GB  Healthy    Boot
 Volume 4     S   Data         NTFS   Partition    380 GB  Healthy

In this case you can see that the Windows on Volume 3 is currently running, there is another one on Volume 2, Boot in the Info column on the far right indicates that this is the current Windows Drive.

you can see the same by open Disk Management:

 diskmgmt.msc

and look for Boot in the Status column of volumes.

When setting up a dual boot Windows you should use descriptive names for each boot entry.