While working with selected cells as shown by @tbur can be useful, it's also not the only option available.
You can use Range() like so:
If Worksheets("Sheet1").Range("A1").MergeCells Then
Do something
Else
Do something else
End If
Or:
If Worksheets("Sheet1").Range("A1:C1").MergeCells Then
Do something
Else
Do something else
End If
Alternately, you can use Cells():
If Worksheets("Sheet1").Cells(1, 1).MergeCells Then
Do something
Else
Do something else
End If