if you don't know what sample prep protocol was used you have to map your reads to a reference genome and look at the sam flags in the bam file. If it is stranded, flags 83, 99, 147 and 163 have the same abundance but in stranded, 2 of these 4 will disappear when you look at either sense or antisense genes only.
I found that the salmon result can depend on whether the reference was assembled as strand-specific or not.
I recommend one of the many great Trinity helper scripts for that, the patterns are very distinct.
You can also check whether your reference (i.e. transcriptome) was assembled as non-stranded although you have stranded libraries :)