I wrote a short species profile for another forum, and thought I could paste the part I wrote about sexing them here. I hope it helps....
Sexing
It's not easy to determine the gender of the Zebra, but it can be done. The fish does need to be sexually mature before it can be identified with some certainty though, and it's important not to look at only one sign, as individual differences can have you easily fooled. A female can display one characteristic typical for a male, and vice versa, but can be a dead giveaway based on another characteristic.
Body shape:
Generally the female wil have a shorter, stouter, body shape - seen from above - than a male. But body shape will depend on how well fed the individual is, and also, the male will occationally display the short and stout body due to the fat reserve he build up when ready to spawn. Body shape is by itself not a sure sign in other words.

Nr1 shows the "normal" body shape of a male. Nr2 shows the "normal" body shape of a female. Nr3 shows a corpulent male. You can see that body shape can not be trusted by itself to identify the gender, but might give a pointer, together with the other characteristics described here.
Head shape

When seen from above, the male has a broader head than the female. The female can also look pinched just where the gills are. This is due to the interopercular odontodes not being as well developed as that of the male, which brings us over to the next sign;
Odontodal growth
Both females and males will have odontodal growth ("hair" or "spikes"), mainly on their pectoral fin, and between the gills. If seen from above, the female very rarely has interopercular growth that extend past where the pectoral fin begins.
This is a picture of a particularly hairy lady:

But when seen from above it's obious that the interopercular odontodes are a lot shorter than those of the male.

The female is to the left, and is the same fish shown as hairy lady above.
Fins
The female has a slightly more crescent shaped pectoral fin than the male. Some people can take one look at the pectoral fin and immidiately know if it's a male or female. I have never been able to do so, and prefer to look at the pelvic fin. The male has a larger pelvic fin than the female. This is also the one sign that has an explanation based on the fish's behaviour. The male will use his pelvic fins to keep the cave clean, and to "fan" the eggs when guarding them. It seem to me a natural consequence that his pelvice fin is larger than the females, as he actually use them for something important. Opinions differ on this though. The first ray of the pectoral fin is also thicker on the male than that of a female.

Female to the left
All the pictures shown here is of a dominant male and a female. There's usually no problem identifying a dominant male, the problem is telling a female and a subdominant male apart, as the characteristics aren't as developed in a subdominant male. He can therefor easily be mistaken for a female. But by using a combination of the characteristics described here it is still possible to tell the genders apart, in most cases, and experience makes the guessing more accurate! I say guessing, because unfortunately there is still en element of guessing involved, that is until the fish is identified through spawning, then one knows for sure
Feel free to correct me if you disagree though
(my server just collapsed, the pictures will be up in a couple of hours)