I am trying to do a simple IF function but don't know how to indicate if a
cell is empty or blank then I want a certain text value returned.

What I am trying is:

=IF('Sheet 1'!A15=IS NULL,"NO","YES")

What I want is if the cell indicated on an other sheet is empty or blank
then the word NO to appear in the formula cell and if it does have text then
YES to appear.  How can I do this?  

Hope I made myself clear enough.  I couldn't figure out how to ask help the
right way to get the correct answer.

Thanks for all of your help!