How do I know if spreadsheet cells are merged using google apps script

Yes, now you can use isPartOfMerge to check.

And for anyone wants to get the value of merged cell:

var value = (cell.isPartOfMerge() ? cell.getMergedRanges()[0].getCell(1,1) : cell).getValue();

Hope it help.

range.isPartOfMerge()

It seems the .isPartOfMerge() method was implemented (11 sept 2016).

So you can check if a range is merged as such:

function testMerge() {
  var ss = SpreadsheetApp.getActiveSpreadsheet();
  var sheet = ss.getSheetByName("Sheet1");
  var range = sheet.getRange(1,1,2); // example for cells A1 and A2
  Logger.log(range.isPartOfMerge());
}

Docs https://developers.google.com/apps-script/reference/spreadsheet/range#ispartofmerge

OLD APPROACH

Posting here another approach that I like more than the accepted answer.

If you try to use GAS to store a value in a cell of a merged range, except the first cell, it will not store it.

As such, we can exploit this and attempt to store the value and check if it was stored, using a function similar to this one:

  function isMerged(range){
  try {
    range.getDataSourceUrl()
  } catch (err) {
    throw "isMerged function only works with valid GAS Range objects"
  }
  var sheet = range.getSheet();
  var rangeData = range.getValues();

  var testValue = "testing merge";

  if (rangeData.length == 1 && rangeData[0].length == 1) return false; // we have a single cell. Can't possible be merged :)

  if (rangeData.length == 1) {
    // We have a single row. Which means we're testing a row with multiple columns
    var mainCell=range.getA1Notation().split(":")[0];
    var rowNumber= sheet.getRange(mainCell).getRow();
    var nextColNumber = sheet.getRange(mainCell).getColumn()+1;
    var row = rangeData[0];
    var oldValue = row[1]; // for testing purposes, we're chosing the next possible column

    sheet.getRange(rowNumber,nextColNumber).setValue(testValue);
    if (sheet.getRange(rowNumber,nextColNumber).getValue() !== testValue) {
      return true;
    } else {
      sheet.getRange(rowNumber,nextColNumber).setValue(oldValue);
      return false;
    };
  } else if (rangeData[0].length == 1) {
    // We have multiple rows and a single column.
    var mainCell=range.getA1Notation().split(":")[0];
    var nextRowNumber= sheet.getRange(mainCell).getRow()+1;
    var colNumber = sheet.getRange(mainCell).getColumn();
    var oldValue = rangeData[1][0]; // for testing purposes, we're chosing the next possible row

    sheet.getRange(nextRowNumber,colNumber).setValue(testValue);
    if (sheet.getRange(nextRowNumber,colNumber).getValue() !== testValue) {
      return true;
    } else {
      sheet.getRange(nextRowNumber,colNumber).setValue(oldValue);
      return false;
    };
  } else {
    // We have multiple rows and multiple columns
    var mainCell=range.getA1Notation().split(":")[0];
    var nextRowNumber= sheet.getRange(mainCell).getRow()+1;
    var nextColNumber = sheet.getRange(mainCell).getColumn()+1;
    var oldValue = rangeData[1][1]; // for testing purposes, we're chosing the next possible row and next possible column

    sheet.getRange(nextRowNumber,nextColNumber).setValue(testValue);
    if (sheet.getRange(nextRowNumber,nextColNumber).getValue() !== testValue) {
      return true;
    } else {
      sheet.getRange(nextRowNumber,nextColNumber).setValue(oldValue);
      return false;
    };
  }

  // if none of these checks worked, something's fishy. Either way, return false
  return false
}

I've ran a series of quick tests and it returned true/false accordingly, but there is a limitation that I didn't have time to cover: If you had a merged range like "A1:F1" and you checked the range "A1:G1" (so, one more column), it will return true, even if G1 is not part of the merged range - because it checks only the next column using as reference the first bound row/column.

For those like me who didn't know the .breakapart() method to unmerge cells: https://developers.google.com/apps-script/class_range#breakApart

Thanks to Henrique for the tip !