See if a ruby string has whitespace in it

If by "white space" you mean in the Regular Expression sense, which is any of space character, tab, newline, carriage return or (I think) form-feed, then any of the answers provided will work:

s.match(/\s/)
s.index(/\s/)
s =~ /\s/

or even (not previously mentioned)

s[/\s/]

If you're only interested in checking for a space character, then try your preference of

s.match(" ")
s.index(" ")
s =~ / /
s[" "]

From irb (Ruby 1.8.6):

s = "a b"
puts s.match(/\s/) ? "yes" : "no" #-> yes
puts s.index(/\s/) ? "yes" : "no" #-> yes
puts s =~ /\s/ ? "yes" : "no" #-> yes
puts s[/\s/] ? "yes" : "no" #-> yes

s = "abc"
puts s.match(/\s/) ? "yes" : "no" #-> no
puts s.index(/\s/) ? "yes" : "no" #-> no
puts s =~ /\s/ ? "yes" : "no" #-> no
puts s[/\s/] ? "yes" : "no" #-> no

some_string.match(/\s/)

It is usually done like this:

str =~ /\s/

You can read about regular expressions here.

"text message".include?(' ') #=> true
"text_message".include?(' ') #=> false

you can use index

"mystring".index(/\s/)