Shu is generally pretty easy to tell grade...
First, any shu that has good qi wins. Most shu do not have any energy. So if a shu is making you feel good, it's probably a winna!
Second, shu should be smooth and thick. Typically, shu just doesn't taste like all that much, nor is it complex tasting. So before taste...is it thick? Is it smooth, like yo mama's chicken soup?
Then there are shu that sports a good coating finish--sweet, earthy, vanilla, milk, fruity, whatever, so that after you swallow, you're left with a pleasant taste of some kind in your mouth.
Next, especially when we're talking about older, more humid-stored shu, does it have a dense and dark aroma?
Lastly, some shu aren't fully fermented, so you'll have to wait 5-10 years for it to be truly bitter-free and ready to drink.
There is, of course, taste. However, most sheng will be tastier than any shu. If you really want taste, and you're not bothered by the green-ness of it all, then drink sheng.
Now, your taste is your own, and you'll juggle the desired qualities above as you sample and buy your way to happiness. Enjoy.