I know you've always wanted to know how to sex an isopod but were afraid to ask. So, here is how you do it and you don't even have to kill the poor creature. First, you catch the isopod between your thumb and 1st finger*. (No, they don't bite and if you hold the isopod gently it will survive the process without any trauma.)
Then, you turn the isopod upside down and look between its hind legs (where else?). If it is a male, there will be at least a pair of long blade-like appendages lying flat near the isopod's posterior end (arrows in the picture below). Those are the modified endopodites of pleopods (inner branches of biramous abdominal limbs) that have evolved to perform the function of sperm transfer. I have magnified pictures of them in this post.
A male Armadillidium nasatum. The arrows point at the pair of modified pleopods.
The females, on the other hand, do not have any external genitalia. So, the absence of modified pleopods means the specimen is a female. It is best to try to sex the largest specimens; I don't know how early during their development the pleopods of males become easily distinguishable from those of females.
Full frontal nudity: this is a female Armadillidium nasatum. Compare with the picture of the male above. Isopods can also be captured and hold with light-weight tweezers.
If you don't have a stereomicroscope, a magnifying glass will help during these procedures.
*I learned this from Stephen Hopkin's A key to the woodlice of Britain and Ireland (Field Studies 7:599-650, 1991; also available as a separate booklet from the NHBS Environment Bookstore).