I would like to know if there is a check for individual achievements, if they are already unlocked, or one that checks all finished achievements (returning a array, which I could check Length for to see if all of them, or X amount, have been finished)?
Unity has no built in achievements code. You write it all yourself. And if you have made some code that handles achievements then being able to see which are achieved at a certain time is a very trivial task.
What have you got so far?