AtomQ

AtomQ[expr] returns True if expr is an atomic type, and False if expr is a full expression.

Attributes[AtomQ] := {Protected}

Simple examples

In[1]:= AtomQ["hello"]
Out[1]= True

In[2]:= AtomQ[(5)/(3)]
Out[2]= True

In[3]:= AtomQ[hello]
Out[3]= True

In[4]:= AtomQ[(a)/(b)]
Out[4]= False

In[5]:= AtomQ[bar[x]]
Out[5]= False