I'm not sure about it, but I think a restaurant owner I once knew told me that the German "Finanzamt" (tax authority) calculates what the average amount of tips

I'm not sure about it, but I think a restaurant owner I once knew told me that the German "Finanzamt" (tax authority) calculates what the average amount of tips should be for any given restaurant based on the data they have (adjusting for location, type, prices). If you report less than that amount in income they'll go after you.