IIRC the survivor skill Know Creature will tell you the level of an NPC. You can save and try it.
As for Pickpocket, I believe standing behind the NPC helps, but I only ever pickpocketed a couple of times (quest related). Have you tried a farmer, or beggar, etc? The people in the quarantine area can be pickpocketed with level 2 of the skill. There is a quest related challenge in the Ducal inn (in Verdistis), if you wish to joint the thieves' guild, which only requires level 1 of Pickpocket (though I don't know if that one can fail).
There is an exploit (almost a cheat) you can use rather than pickpocket, but it will take all of the challenge, and most of the fun, out of stealing.
<span class='standouttext'>Spoiler : </span><span class='spoiler'>If you open a container in your inventory, you can drag and drop items into it, and it is not considered stealing (a couple NPCs get annoyed simply moving their stiff, though). It will be seen as stealing the first time you remove the items from the container, though, so make sure you are alone. A sack of grain is the lowest weight container you can use before getting into the thieves' guild.</span> (highlight the text to read)
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