Looking through the help, I see that the install defaults to your processor architecture; but if you have a 64-bit OS and only 32-bit package is available, it will fall back. So how can you tell which one you're getting (ideally without installing)? Also, will x86 package names always end in x86?
Using --checksum64 somehow might work, but I haven't been able to get that to work yet. FWIW, there's a force x86 install option, but there doesn't seem to be one to force x64.
I don't love this answer, but it's a start. Run choco install [package], and it will download the binar(y|ies) and pause to ask if you want to actually install them. I'm not sure of the exact location it will drop them, but I just searched my AppDAta for the package.
Directory of C:\Users\[username]\AppData\Local\Temp\chocolatey\NuGetScratch\0b0d34f0-3921-4e7b-8997-68b940ecb090\fh12npqr.j5j\tools
09/19/2019 10:36 AM 41,561,144 vcxsrv-64.1.20.5.1.installer_x64.exe
09/19/2019 10:36 AM 40,632,035 vcxsrv.1.20.5.1.installer_x32.exe
2 File(s) 82,193,179 bytes
Fortunately for me, I can clearly see that I have both options. If only one is there, I guess you just have to probe the binary to see what it is.