If you have ever applied in the past for an immigration benefit, you most likely have an A#. Not all benefits will provide you with a number though.
And sometimes the number you are assigned for one kind of benefit (a EAD based on a J-2, for example) will not be the same number assigned later if you apply for I-140 or I-485.
If you ever have gotten a receipt notice from USCIS in the past, see the small box on the upper right hand side of the notice (2 boxes down on the right) where it says "Applicant". You will see your name there and perhaps a sequence of numbers that are preceded by "A". That would be your A number. If this does not apply to you at all, don't leave the I-140 and I-485 field blank, instead type or print "None".