There's no sure-fire way of telling, sadly - and thankfully there aren't too many around (though any is too many) - but it's always wise to look out for phrases like 'totally remastered for Blu-ray" and "restored for HD" on the box, as they'll typically indicate some thought's gone into the process.
We had this with DVD - some of the first releases were truly shocking (look how long it took to get a decent copy of Blade Runner) before the studios hit on the idea of 'special editions' that were remastered.
It's one of the reasons i"m still buying very few Blu-rays - I rent from Lovefilm, instead.
Having said all that, when you get a great Blu-ray - for example, I watched 3:10 from Yuma recently; awesome picture, 7.1 PCM sound - it takes home cinema to a totally new level of excellence.
Clare Newsome is Editor-in-Chief of What Hi-fi? Sound and Vision