I don't think you can... I meant that: it would only be possible
if steam/gog etc publicly displayed some kind of sales stats for each game, but as far as I'm aware, they don't, & probably won't.Here
is a very interesting article from a developer who has published their game on various digital distribution platforms. It gives various stats & their experiences, & thoughts on each.
This place here
is also quite interesting... it gives various summaries & breakdowns of units sold of Deponia. I don't know if it is accurate/official or not though.