Smart Tag

Check your (Options, Formulas tab, "Enable background error checking").
The specific background checks that can be performed, are called Error Checking Rules.
You can decide which rules you want to include and which ones you don't from the Excel Options dialog box.

The screen shot above shows the default settings.

Cells containing formulas that result in an error

This rule will identify cells containing formulas that result in errors.
Results in an Error

Inconsistent calculated column formula in tables

This rule will identify cells in a calculated table column that are different to the others in that column.
Calculated Table Columns

Cells containing years represented as 2 digits

This rule will identify cells containing dates that have been entered as text with only a 2 digit year.
Text with 2 Digit Year

Numbers formatted as text or preceded by an apostrophe

This rule will identify cells containing numbers that have been formatted as text or have been preceded by an apostrophe.
Number Stored As Text

Formulas inconsistent with other formulas in the region

This rule will identify cells containing formulas that are inconsistent with other formulas in the same region.
Inconsistent Formula

Formulas which omit cells in a region

This rule will identify cells containing formulas which do not include all the adjacent cells in the same region.
Omits Adjacent Cells

Unlocked cells containing formulas

This rule will identify cells containing formulas that will be unprotected should the worksheet be protected.
Unprotected Formula

Formulas referring to empty cells

This rule will identify cells containing formulas that refer to empty cells.
Refers to Empty Cells

Data entered in a table is invalid

This rule will identify cells containing data that is not valid.
Data Validation Error

Misleading number formats

This rule will identify cells containing formulas that have a different number format to the one applied to the destination cell.
Misleading Number Formats