The SATA controller in the Dell Latitude E5420 are SATA III. I have a Crucial C300 128 GB SATA III SSD installed in my E5420 with the Intel HM65 Sandy Bridge chipset and the read speed of my SSD tops at 370 MB/s in the ATTO 2.46 4K read test. This is clearly SATA 3 speed (a SATA II controller could not go beyond 285 MB/s in the SSD read test)
Besides, there are no SATA II or SATA III connectors. They are all the same. There aren't even SATA III cables, there also all the same. What does make the difference, is the the SATA controller. Old chipsets had SATA I or II controllers and with the introduction of the Intel Sandy Bridge chipsets, the SATA controller now support SATA III 6.0 Gbps transfer (which is of course backwards compatible with SATA II and even SATA I).