How do I determine a mapped drive’s actual path?

I expanded on ibram’s answer and created this class (which has been updated per comment feedback). I’ve probably over documented it, but it should be self-explanatory.

/// <summary>
/// A static class to help with resolving a mapped drive path to a UNC network path.
/// If a local drive path or a UNC network path are passed in, they will just be returned.
/// </summary>
/// <example>
/// using System;
/// using System.IO;
/// using System.Management;    // Reference System.Management.dll
/// 
/// // Example/Test paths, these will need to be adjusted to match your environment. 
/// string[] paths = new string[] {
///     @"Z:\ShareName\Sub-Folder",
///     @"\\ACME-FILE\ShareName\Sub-Folder",
///     @"\\ACME.COM\ShareName\Sub-Folder", // DFS
///     @"C:\Temp",
///     @"\\localhost\c$\temp",
///     @"\\workstation\Temp",
///     @"Z:", // Mapped drive pointing to \\workstation\Temp
///     @"C:\",
///     @"Temp",
///     @".\Temp",
///     @"..\Temp",
///     "",
///     "    ",
///     null
/// };
/// 
/// foreach (var curPath in paths) {
///     try {
///         Console.WriteLine(string.Format("{0} = {1}",
///             curPath,
///             MappedDriveResolver.ResolveToUNC(curPath))
///         );
///     }
///     catch (Exception ex) {
///         Console.WriteLine(string.Format("{0} = {1}",
///             curPath,
///             ex.Message)
///         );
///     }
/// }
/// </example>
public static class MappedDriveResolver
{
    /// <summary>
    /// Resolves the given path to a full UNC path if the path is a mapped drive.
    /// Otherwise, just returns the given path.
    /// </summary>
    /// <param name="path">The path to resolve.</param>
    /// <returns></returns>
    public static string ResolveToUNC(string path) {
        if (String.IsNullOrWhiteSpace(path)) {
            throw new ArgumentNullException("The path argument was null or whitespace.");
        }

        if (!Path.IsPathRooted(path)) {
            throw new ArgumentException(
                string.Format("The path '{0}' was not a rooted path and ResolveToUNC does not support relative paths.",
                    path)
            );
        }

        // Is the path already in the UNC format?
        if (path.StartsWith(@"\\")) {
            return path;
        }

        string rootPath = ResolveToRootUNC(path);

        if (path.StartsWith(rootPath)) {
            return path; // Local drive, no resolving occurred
        }
        else {
            return path.Replace(GetDriveLetter(path), rootPath);
        }
    }

    /// <summary>
    /// Resolves the given path to a root UNC path if the path is a mapped drive.
    /// Otherwise, just returns the given path.
    /// </summary>
    /// <param name="path">The path to resolve.</param>
    /// <returns></returns>
    public static string ResolveToRootUNC(string path) {
        if (String.IsNullOrWhiteSpace(path)) {
            throw new ArgumentNullException("The path argument was null or whitespace.");
        }

        if (!Path.IsPathRooted(path)) {
            throw new ArgumentException(
                string.Format("The path '{0}' was not a rooted path and ResolveToRootUNC does not support relative paths.",
                path)
            );
        }

        if (path.StartsWith(@"\\")) {
            return Directory.GetDirectoryRoot(path);
        }

        // Get just the drive letter for WMI call
        string driveletter = GetDriveLetter(path);

        // Query WMI if the drive letter is a network drive, and if so the UNC path for it
        using (ManagementObject mo = new ManagementObject()) {
            mo.Path = new ManagementPath(string.Format("Win32_LogicalDisk='{0}'", driveletter));

            DriveType driveType = (DriveType)((uint)mo["DriveType"]);
            string networkRoot = Convert.ToString(mo["ProviderName"]);

            if (driveType == DriveType.Network) {
                return networkRoot;
            }
            else {
                return driveletter + Path.DirectorySeparatorChar;
            }
        }           
    }

    /// <summary>
    /// Checks if the given path is a network drive.
    /// </summary>
    /// <param name="path">The path to check.</param>
    /// <returns></returns>
    public static bool isNetworkDrive(string path) {
        if (String.IsNullOrWhiteSpace(path)) {
            throw new ArgumentNullException("The path argument was null or whitespace.");
        }

        if (!Path.IsPathRooted(path)) {
            throw new ArgumentException(
                string.Format("The path '{0}' was not a rooted path and ResolveToRootUNC does not support relative paths.",
                path)
            );
        }

        if (path.StartsWith(@"\\")) {
            return true;
        }

        // Get just the drive letter for WMI call
        string driveletter = GetDriveLetter(path);

        // Query WMI if the drive letter is a network drive
        using (ManagementObject mo = new ManagementObject()) {
            mo.Path = new ManagementPath(string.Format("Win32_LogicalDisk='{0}'", driveletter));
            DriveType driveType = (DriveType)((uint)mo["DriveType"]);
            return driveType == DriveType.Network;
        }
    }

    /// <summary>
    /// Given a path will extract just the drive letter with volume separator.
    /// </summary>
    /// <param name="path"></param>
    /// <returns>C:</returns>
    public static string GetDriveLetter(string path) {
        if (String.IsNullOrWhiteSpace(path)) {
            throw new ArgumentNullException("The path argument was null or whitespace.");
        }

        if (!Path.IsPathRooted(path)) {
            throw new ArgumentException(
                string.Format("The path '{0}' was not a rooted path and GetDriveLetter does not support relative paths.",
                path)
            );
        }

        if (path.StartsWith(@"\\")) {
            throw new ArgumentException("A UNC path was passed to GetDriveLetter");
        }

        return Directory.GetDirectoryRoot(path).Replace(Path.DirectorySeparatorChar.ToString(), "");
    }
}