How to check if a class already exists in Ruby

How do I check if a class already exists in Ruby?

My code is:

puts "enter the name of the Class to see if it exists"   
nameofclass=gets.chomp  
eval (" #{nameofclass}......  Not sure what to write here")

I was thinking of using:

eval "#{nameofclass}ancestors.     ....."

How to check if a class already exists in Ruby