SQL Server check resultant data type of expression

Normally I would put the data type into a temp table and inspect the table column type, e.g.

select 1.0 N into tbl

sp_help tbl

Column N then reveals the data type of the expression 1.0. (This is a only simple example)

There is a SQL function to inspect the data type of an expression directly, but the name escapes me right now.

What is the name of this function?