How to check from .net code whether "Trust access to the VBA project object model" is enabled or not for an Excel application?

The short answer is that you cannot directly access this setting using the Excel object model (i.e. through PIAs). However, instead, you can check this setting from the registry in the following location (here I assume that you're using Office 2007 - version 12.0):

HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Excel\Security\AccessVBOM

this is a DWORD that will be 0 or 1 depending on whether the "Trust access to VBA Object Model" is enabled.

However, this setting can be overriden by another registry key located at:

HKEY_LOCAL_MACHINE\Software\Microsoft\Office\12.0\Excel\Security\AccessVBOM

this is again a DWORD, however, if this value is 0 this means that no matter what the HKCU value is set to, then access to the VBOM will be denied. If the value in HKLM is 1 or missing, then the HKCU key will control the access to the VBOM.

Therefore, all you need to do is to check these two keys via the Registry methods in .NET.

This worked for me

Function VBATrusted() As Boolean
    On Error Resume Next
    VBATrusted = (Application.VBE.VBProjects.Count) > 0
End Function

Private Sub Workbook_Open()
    If Not VBATrusted() Then
    MsgBox "No Access to VB Project" & vbLf & _
      "Please allow access in Trusted Sources" & vbLf & _
      "File > Options > Trust Center > Trust Center Settings > Macro Settings > Trust Access..."
    End If
End Sub

Source https://www.mrexcel.com/forum/excel-questions/659774-checking-if-trust-access-visual-basic-project-ticked.html