Apply conditional formatting with custom formula
This can be applied even to a range of headers such as A1:E1; the formula, entered as above, will be interpreted in a relative way for other columns.
not(isnumber(...)) is true if the entry is not a number
N converts logical true/false to integers 1 or 0
sum adds these.
arrayformula makes sure the entire range is handled
- the sum is equal to 0 if all entries are numbers.
Dates and times are considered numbers, because internally, they are. For example, the date 05/26/2016 is the number 42516 formatted as a date.