How to judge the character and sign of a dislocation?

Question

I do understand that $ \mathbf b||\mathbf t$ for screw dislocation but I am unable to judge the sign of the dislocation. Again if $\mathbf b \perp \mathbf t$ how do I judge its sign? Any formula which explains the sign and magnitude (to some extent) will be highly appreciated. Here, $\mathbf{b}$ is the Burger's vector and $\mathbf{t}$ is the tangent vector.