Mangosteens are also among my favourite fruits and there are those who will wish me intense harm when I say that I've only ever had them fresh and it's the season now.
We pick out our selection here both in markets and supermarkets.
As to testing their freshness, I rely a lot on the old "know your supplier" principle, but in addition employ a squeeze and scratch method. A squeeze should react as in the post above. If more than usually suspicious, a light scratch should reveal that the purple skin is still slightly moist (leaking purple juices, which are sadly beautiful but inedible.) A dry skin is not a good sign. Nor would be a mushy one.