It will be very helpful to show the column type in RStudio data viewer. Sometimes it's not obvious like shown numbers can be either numeric or characters. I just spend some extra time because I didn't realize the columns are actually characters, not numbers.
A small tag or icon in column header should be enough.
If you hover with your mouse over the column name, you'll see a tooltip with the variable type and some additional info depending on the type of variable:
That is helpful, but one problem is that user often didn't realize there was a type problem so he will not check it by hover. If he want to check he will check with all kind of methods.
The problem here is that the problem better show themselves instead of let user to check it specifically.