Hi Karsten,
Victor's suggestion is the best as I know.
But if your portal is under 10.7, probably try getItem or getLayerList methods in utils module:
https://developers.arcgis.com/javascript/3/jsapi/esri.arcgis.utils-amd.html
layers can be compared between response.itemData vs this.map.itemInfo.itemData.
item modified date can be compared between response.item.modified and this.map.itemInfo.item.modified.
