Well, good measure is how big the queues are outside the shop!
Seriously though, word of mouth, online reviews, or better, buy a small amount first and try it out. You can also ask them what's in it.
And an independent butcher has a reputation to maintain. If word gets round that he is selling poor quality goods, his business is finished. The business model of independents is better quality products with personal service.
I occasionally get a leg of lamb from my local farm shop, it costs a bit more, but he'll bone and roll it for me, or bone and dice it, at no extra cost, and far netter and quicker than I could.