If you look at the back, one port will say "zip", this is the side that goes to the computer. The second port will have either a small printer, or the SCSI logo. If it has the SCSI logo, it's SCSI. If it had a small printer, it's parallel. (The SCSI model also has to small switches, one to set the SCSI ID, and one to turn termination on or off.)
You can see pictures on Iomega's support site. It shows every ZIP drive they made. The fourth one down is parallel, the fifth is SCSI. (A second pair, the 100 MB models, are located further down the page as well, although they look identical.) Second to last is the rare "ZipPlus" which would automatically switch between SCSI mode and parallel mode depending on what you had it plugged in to. Notice that it says "AutoDetect" instead of "zip", and has both the SCSI logo and printer logo, as well as only one switch. Again, this one is really rare.