For thin lenses we can use the lens maker's equation:
Where $n$ is the index of refraction of the material, $R_1$ is the radius of curvature of the side the light hits first, and $R_2$ is the radius of curvature of the side the light hits last.
For each $R$, the convention is such that we make $R>0$ if the light hits the curved surface before the center of curvature, and $R<0$ if the opposite is true.
Therefore, in your example with light approaching from the left, both $R_1$ and $R_2$ should be taken to be positive if you are using this formula. This results in a positive focal length.
I think a good rule of thumb though is that if the lens is thicker in the middle than on the edges it is a converging lens, and hence has positive focal length.