I don't think all soldiers carry atropine needles. Only soldier in a high risk areas or jobs would have it(special forces come to mind).
Yes, they all carry atropine (in the form of NAAK kits), just like they all carry gasmasks. Now of course that only applies to situations where the use of chemical/bioligical weapons is possible/likely, such as in a wartime environment. For example, all of the soldiers and marines in the recent Iraq war carried gasmasks and atropine needles (not sure if they still do, but its possible). That is, ALL of the forces, not just those on the frontlines. Half my reserve unit ran supply ops during the war (not me though
), and they all had gasmasks and atropine, as well as full protective MOPP suits.
Also you would not have time to get a needle of atropine from the first aid tent. It needs to be injected in seconds.
Which is why they all carry the injections individually.
More than likely the only way a normal soldier would know if they were around bio- or chemical weapons is when some or all the group start reacting to the agent.
Or if they see an aircraft make a low pass overhead (like a crop sprayer), or if they see incoming shells that don't explode. Also, there is a certain detector tape (can't remember the name--its "m" followed by some number
) that is strapped around sleeves/whatever, which changes color when it comes in contact with certain agents.