Certified GKH Gardening Expert
It depends on the variety of the citrus tree, but both lemon and lime trees can have thorns. Meyer lemons and Tahiti limes are commonly seen with thorns. Generally, these are not as prominent in older, larger trees. Most likely, you'll just have to wait until the tree produces fruit to know what you have. Keep in mind that limes are picked prior to maturity, while still green--otherwise, they will become yellow and over ripe.