Consult a recording industry publication. Many companies have built their fortunes on chart publication and have transitioned to publishing digital music sales. These companies have new print or Web editions available daily, weekly or monthly.
Visit a record label's Web site. Labels from large multinationals to "Indies" often publish sales figures for their own digital singles. If you know the label to which a specific artist belongs, you can check the label's site or contact them for sales figures.
Visit the site of a digital music seller. Many companies from online music sellers to music subscription services list the sales of their singles.
Visit the Web site of the specific artist whose sales you wish to check. Many recording artists feature the current sales figures for their singles on their official Web sites.