I cut Ellie's nails. They have never bled. Just take the tips off. As soon as you see a dark dot in the middle of the powdery white color (after you make your first cut), stop. That is the quick. It is actually the cutest thing. When I do Ellie's I clip a couple times and decide to clip once more (further down). As soon as I start to squeeze the clippers she cries (she knows I'm about to hit her quick and won't let me). That is how I can tell.
Of course next time I do them I will probably make her bleed now that I wrote this.