As he passed them, Eldrian realized that just seeing their mana wasn't enough, so he also used Observe to make sense, and quantify, what he saw. Luckily the guards were only Tier 4 and didn't have as strong senses as Old Sword or Eldrian would quickly have landed in trouble.
There was no rule for using Observe on others, but it would certainly make the guards dislike him. They might even opt to beat him up, if not for the fact that they must stay at their posts.
The guards' mana was similar to the others Eldrian had looked at, however, they had two mana pools. Those in the fields had been a mix of demi-humans, humans, and centaurs. They all had a single mana pool at their diaphragm, for the centaurs this was on their human half.
