* He is a military man. Holmes deduces this from Watson's "military carriage," his "strong, regular features," and his "sunburnt face."
* He has recently been injured. Holmes notices the empty sleeve and the "freshly healed scar" on Watson's arm, suggesting a recent injury.
* He is a man of action. From Watson's "alert, eager face" and his "sharp, decisive manner," Holmes deduces that he is a man of action, perhaps someone who is used to making quick decisions.
Holmes doesn't specifically guess about Watson's profession or his personality beyond these initial observations. However, these deductions already provide a glimpse into Holmes's sharp powers of observation and deduction.