Check the diplomacy section in the research tree summary:
on you top bar.
You can open the various skills to find out what they do and the ones with doves next to them unlock a diplomatic unit - they also display the unit build costs
e.g. For a human:
       | Theft | 400 | 8hr 53m 20s | Requires Consulate at lvl 9 | |
    | Your diplomatic skills are advancing apace! This technology allows you to train thieves to steal resources from enemy cities. Basic thieves can steal Wood, Clay, Iron, Stone, Food, Beer & Books. Enables Units:Burglar Build Costs: | Upkeep Costs: |  | | 2 |
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       | Thiefmastery | 850 | 12hr | Requires Consulate at lvl 11 | |
     | An extension to the Theft skill, this technology unlocks the Advanced thief units. In addition to the basic resource types, Advanced Thieves can also steal advanced resource types, including gold! Unfortunately, however, even the best thief has trouble making off with something as large as a Siege Block. Enables Units:Master Thief Build Costs: | Upkeep Costs: |  | | 4 |
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