$ head -1 structureinput.tsv | tr '\t' '\n' | wc -l
This will return a value to the screen which is also the number of loci in your dataset (assuming the dataset is formatted with two rows per individual - which is the way Stacks outputs the file).
head -1: produces the first line in the file
structureinput.tsv: is the structure formatted input file
| : pipes the output of the first command to the second command
tr : is the "translate" command
'\t' : is what identifies tab separator in the output from the head -1 command
'\n' : is the identifier to say put each tab separated value into a new line
| : pipes the output from the translate command to the next command
wc -l : counts the number of lines in the new output
You can also just send the output to a file that you can look at in TextWrangler or probably in Excel if it is not too large.
$ head -1 structureinput.tsv | tr '\t' '\n' > snps.txt
This command does the same thing as the one previously but instead of counting the number of lines it simply outputs the file "snps.txt" which you can then open and look at to see if it was translated properly.