INTERNAL Consistency

Good day all,
I have lake waterlevel data from four gauging station. What are some of the methods I can use to detect outliers in the data (not boxplots), that will also allow me to detect the internal consistency of the data. the problem is all the stations have missing values.

You might want to check ?IQR and ?quantile.

I was hoping for something that will indicate the daily variation found in the data. And should a value fall out of the range of all the values, it should be flagged.