Sometime an object has attribute names for identification of the value with names instead of index. There are time you need to extract the names of a vector or object. The easiest way is to do:
library(data.table)
dt <- cars[1,]
setDT(dt)
str(dt)
dt[, dist := as.character(dist)]
The above is just to make a toy data and convert column dist to character. In
case we want to know later which column or names that we have converted to class `character`.
bb <- dt[, lapply(.SD, class)]
> names(bb)[bb == "character"]
[1] "dist"
This can then be implemented programmatically. Of course you can do str(bb)
but that is not the point here and str() is only suitable for viewing purposes.