Form validation for field create with jquery

Hi at all sorry for english, i am doing an invoice .
i have one row with input text  article name= 'article[1]' and input text quantity name= 'quantity[1]' .
With jquery i create a new row clicking a button for every click i add a row

the name of input are increase (f.e. article[2] and quantity[2] , article[3] and quantity[3])

How can i validate this value ,i want set required for this field ...