C# - How to tell if system has virtual memory / page file on?

You'll need to add reference to System.Management beforehand.

AllocatedBaseSize will show the current page file size in MB

using (var query = new ManagementObjectSearcher("SELECT AllocatedBaseSize FROM Win32_PageFileUsage"))
        {
            foreach (ManagementBaseObject obj in query.Get())
            {
                uint used = (uint)obj.GetPropertyValue("AllocatedBaseSize");
                Console.WriteLine(used);
            }
        }

While MaximumSize will show the maximum page file size in MB, if the user set the maximum size (if the system managed it, the query won't return anything).

using (var query = new ManagementObjectSearcher("SELECT MaximumSize FROM Win32_PageFileSetting"))
        {
            foreach (ManagementBaseObject obj in query.Get())
            {
                uint max = (uint)obj.GetPropertyValue("MaximumSize");
                Console.WriteLine(max);
            }
        }

If the AllocatedBaseSize is less than what your app will use and the MaximumSize is large enough for your app (or it's system managed), you'll need to consider the edge case where the storage is not enough for Windows to grow the page file. Even if there is enough space in the beginning, user could be downloading a large file on other program or rendering a large video while running your app. Consider offering 'low storage' mode where your app may run slower but don't consume as much memory.

Whilst I don't have a complete working solution for you, I think the information you are after can be retrieved from the Win32_PageFileUsage WMI class. The AllocatedBaseSize property should contain the information you are after:

AllocatedBaseSize

Data type: uint32

Access type: Read-only

Qualifiers: MappingStrings ("Win32API|MEMORYSTATUS|dwTotalPageFile"), units ("megabytes")

Actual amount of disk space allocated for use with this page file. This value corresponds to the range established in Win32_PageFileSetting under the InitialSize and MaximumSize properties, set at system startup. Example: 178