Two quick functions to return a system's firmware interface.

The first function uses a C# method to determine and return the type of firmware interface a system is using.

The second function uses a compiled Win32 API to test a system's firmware, returning a boolean value of "True" or "False" depending on whether or not UEFI is detected.

Code:

<#
    .NOTES
    ===========================================================================
     Created with:     SAPIEN Technologies, Inc., PowerShell Studio 2018 v5.5.150
     Created on:       4/19/2018 10:15 AM
     Created by:       BenTheGreat
     Filename:         DetectSystemFirmware
    ===========================================================================
    .DESCRIPTION
        Two functions, one returning a string value and the other a boolean value, to retrieve a system's firmware interface (BIOS).
#>

Function Get-FirmwareInterface
{
<#
    .SYNOPSIS
        Retrieves the system's firmware interface (BIOS).
 
    .DESCRIPTION
        Retrieves the system's firmware interface (BIOS) and returns a string indicating the firmware type: Legacy BIOS, UEFI or Unknown.
 
    .EXAMPLE
        PS C:\> Get-FirmwareInterface
#>
    [CmdletBinding()]
    [OutputType([System.String])]
    Param ()

    Begin
    {
        Add-Type @'
using System;
using System.Runtime.InteropServices;
public class FirmwareInterface
{
    [DllImport("kernel32.dll")]
    static extern bool GetFirmwareType(
        ref uint FirmwareType);
    public static uint GetFirmwareType()
    {
        uint firmwaretype = 0;
        if (GetFirmwareType(ref firmwaretype))
            return firmwaretype;
        else
            return 0;
    }
}
'@
    }
    Process
    {
        If ([FirmwareInterface]::GetFirmwareType() -eq 1) { $FirmwareInterface = "Legacy BIOS" }
        ElseIf ([FirmwareInterface]::GetFirmwareType() -eq 2) { $FirmwareInterface = "UEFI" }
        ElseIf ([FirmwareInterface]::GetFirmwareType() -eq 3) { $FirmwareInterface = "Unknown" }
        Return $FirmwareInterface
    }
}

Function Test-UEFI
{
<#
    .SYNOPSIS
        A compiled Win32 API that tests the system's firmware for a UEFI interface.
 
    .DESCRIPTION
        Tests the system's firmware for a UEFI interface and returns a bool value of True if a UEFI interface is detected and False if a Legacy BIOS is detected.
 
    .EXAMPLE
        Test-UEFI
        If (Test-UEFI) { Write-Output "UEFI Interface detected." } Else { Write-Output "Legacy BIOS Interface detected." }
#>
    [CmdletBinding()]
    [OutputType([System.Boolean])]
    Param ()
 
    Begin
    {
        Add-Type @'
using System;
using System.Runtime.InteropServices;
public class TestUEFI
{
    [DllImport("kernel32.dll", SetLastError = true)]
    static extern UInt32
    GetFirmwareEnvironmentVariableA(
        string lpName,
        string lpGuid,
        IntPtr pBuffer,
        UInt32 nSize);
    const int ERROR_INVALID_FUNCTION = 1;
    public static bool IsUEFI()
    {
        GetFirmwareEnvironmentVariableA("", "{00000000-0000-0000-0000-000000000000}", IntPtr.Zero, 0);
        if (Marshal.GetLastWin32Error() == ERROR_INVALID_FUNCTION)
            return false;
        else
            return true;
    }
}
'@
    }
    Process
    {
        [TestUEFI]::IsUEFI()
    }
}
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To run, simply enter Test-UEFI or Get-FirmwareInterface