violas have different proportions than a violin or cello, and a string bass is unmistakable being from the viol family instead of the violin family. (Arguable, but that's the side I take based on neck shape being tapered rather than straight in, difference in tuning, and the proportions that are more in line with viols, despite the lack of frets and use of only four strings.) String basses also use gear pegs. And yes, the presence of a shoulder rest is a dead giveaway. The viola looks squatter than the violin.
The other giveaway is in the bow. A viola bow has a rounded back of the frog compared to the squared off violin frog. This is more tradition as far as I can tell. The viola bow is heavier.
This is a student violin. Hard to tell the actual size with lack of proportions, but I wouldn't be surprised if it is a 3/4 size rather than full size due to the presence of beginner's tape.