Full Idea
A future accelerator might create 'stringballs', when two strings slam into one another and, rather than combining to form a stretched string, make a tangled ball. Finding them would prove string theory.
Gist of Idea
String theory might be tested by colliding strings to make bigger 'stringballs'
Source
New Scientist writers (Why the Universe Exists [2017], 08)
A Reaction
This is the only possible test for string theory which I have seen suggested. How do you 'slam strings together'?
Book Reference
New Scientist writers: 'Why the Universe Exists' [John Murray 2017], p.158