No value has been assigned to the variable x:

Assign a value:

Now ValueQ gives True:

Define a function:

ValueQ[f[x]] is True:

Check immediate and delayed values of symbols:

Check function definitions:

Expressions with head f give True whether or not they actually change:

Use a method that depends on the particular arguments:

ValueQ[LocalSymbol[…]] gives True if and only if the local symbol has a value:

By default, ValueQ[symbol] only checks symbol for OwnValues, not other kinds of values:

ValueQ[x] is True because a value was assigned directly to x:

ValueQ[f] is False because f itself was not assigned a value:

For more general expressions, ValueQ checks for any kind of symbol definition:

ValueQ is HoldAll:

If x is allowed to evaluate prior to entering ValueQ, the result for y will be given:

Pi evaluates to itself:

Use NumericQ to find out whether there is a numerical value defined:

ValueQ checks both user and built-in functions for definitions:

While f has no definitions, Plus does:

ValueQ is HoldAll:

Here x is evaluated before ValueQ sees it:

Use Unevaluated to preserve the HoldAll attribute: