This question is an exact duplicate of:
When I'm looking at an answer to a question that I asked, I find it hard to tell whether I've accepted that answer.
The difference is clear when they're placed next to each other, but not in isolation. The dark gray (#817f84) of the unchecked accept mark is more pronounced against the background than the lighter gray (#b5b5b5) of the inactive voting button, so if one doesn't a priori expect the check mark to be green, it might well mean that the check mark is active. Conversely, the green (#459718) is a bit washed out, so it isn't completely obvious that the check mark is active.
I haven't had this problem on other Stack Exchange sites. For example, on a beta site, it's
The unchecked mark is the same shade of light gray as the inactive voting button, and the checked mark is a more pronounced green, so each is obvious in isolation.
In addition, there's a positioning glitch with the check mark on English.SE: it moves a few pixels towards the left when you mark an answer as accepted. Neither positioning of the check mark looks wrong to me in isolation, but it's weird that the position depends on the state.
Please tweak the colors of accept marks to make it easier to tell whether they're active or not. Less importantly, please settle on one vertical alignment.