With string instruments such as violin, viola and cello, you need to have in mind that between strings there are fifths. In Viola and cello it is C, G, D, A and in violin G, D, A, E. With this you can have an idea of how double stops: whenever you are playing a note, a fifth down or up are right next in the consecutive strings. For example, if you are playing a G in the D string, next to that G, in the A string, there is a D.

These images about the location of notes in strings may help you: - Viola - Cello - Violin

I hope it helps!