There's two ways you can do this.
1) Select column A then select Find in the Edit menu. Type in the first 6
digits of a number in column B and click the Find Next button. The first
cell with a matching number in column A will be highlighted. If a match is
found, close the Find dialogue box and edit the number in column B. If no
match is found, a message is displayed. Close the message and change the
'Find' value to next column B number to be checked.

2) Select column A then select Filter Auto Filter in the Data menu. This
puts a filter button (down-arrow) at the top of the column which, when
clicked, lists the contents of the column in numerical and alphabetical order
in a drop-down box which can be scrolled. This makes it easy to manually
search for a match of the numbers in column B. When you click on another cell
(e.g. to edit a matching number in column B), the drop-down box will close.
So you'll have to click the filter button again to search for the next number.

"Secret Squirrel" wrote:

I'm trying to compare to columns to see if they have any of the same entries
but the data is not totally exact in both columns.

For example I have in column A a value of "211000" and in column B I have a
value of "211000-1". How can I have it compare the two and only look at the
first 6 characters of column B? I want to put a "1" in next to the column B
value to show that it exists in column A. Now Column A has 538 entries in it
and Column B has 24 in it. So I want it to search the entire listing in
column A and have it put a "1" next to the value in column B to show that it
does exist in column A. Hope this all makes sense. :)