Slice the cucumber in half length-wise. A fresh cucumber will show firm seeds and watery, clear flesh. If the cucumber has gone bad, it will have a mushy texture and watery ooze.
Cucumber molds appear as fine threads and spores and are hard to spot with the naked eye. Mold is an indication that your cucumbers had already turned bad at least a day ago, so if you see mold, throw away the whole cucumber right away.