I know there is a "estimate score " function built into the sidebar, but it seems to be a bit different to the one used after both players have passed. I don’t want to have to count every space after someone resigned, just to find out if I deserve that resignation or not.
Unfortunately not, and chances are it wouldn’t be helpful. Unless they resigned instead of passing (which is rare), the game likely wasn’t finished and you can’t accurately score an unfinished game. The closest you could do is put the game into an SGF editor that scores it for you, finish the game logically, and have it score it for you.
However, the best option would be to count it yourself. Counting is really good practice. (Full disclosure, I still kinda suck at counting mid-game, and it leads me to make unnecessarily risky plays and lose games I could have won if I had counted more accurately/often and just stabilized the territory I already had.)
Get yourself the KGS client (https://www.gokgs.com/download.jsp or get a JAR file from https://www.gokgs.com/). It has really good score estimator and SGF editor. You need Java. I have it installed just for that.
Last week I was watching an interesting game between two low Dan players, and the one I thought was ahead resigned (most of the major territory was already decided). I clicked on the score estimator (not knowing whether it would work for the audience, but what the heck…), and it showed the person who resigned as 3 points ahead (the scores seemed plausible to my novice eye, meaning it wasn’t just captures and a tiny bit of territory). For what it’s worth…