A good indication today is Google research itself, if you need to know it on the fly.
You could use the key words "a an unforgettable", it will show '"did you mean?'' and suggest the right one.
With "a an university" it displays a lot of grammar sites, at least you can enter one and see the answer. You could use the Libre Office´s Writer and it would say that "an university" is wrong, it says "wrong article" and suggest the right way.
I mean, if the Writer check spelling is set to "English'', not Portuguese.