Well, technically speaking, you assess via a kind of series of indicators.
Person says they want to kill themselves.
With a child I would always be concerned anyway, because they don't generally (unless a significant adult or experience has made them think of suicidal language as a normal way of getting attention...and then it's potentially serious if they have such an example.)
Do they have a plan to do so?
If they have a plan, do they have the means?
Have they been doing anything like give stuff away, saying good bye to people/pets?
Has there been a change in their behaviour or affect?
Are they doing the things they normally do with the same enthusiasm?
Do they "feel" right to you, if you know them well?
Are they feeling hopeless about whatever is worrying them, life generally?
If they have a realistic plan and means, TAKE AWAY THE MEANS and bloody well watch them.
That is a true crisis situation, and one where responsible adults need to get the kid help.
I'd most definitely consider a professional.
I'd also be finding out from child what is leading to them feeling this way?
Often, just talking about the issues is the key...especially if the child is catastrophising a normal life event, or has a specific worry.
It's thought that a number of childhood "accidents" can be attributed to suicidal behaviour.
Hell, I tried to kill myself when I was seven or eight (low lethality, but I sure as hell needed help because of an awful family situation...of course, being as how the family situation was awful, I didn't get said help!)
It sounds as though this kid is feeling pretty damn bad, and it is necessary to take their distress very seriously.
Guns and boys are a bad combination. Boys tend to make suicidal gestures or attempts less often than girls, but they tend to be more lethal when they do it.
Really, only lots of careful discussion and assessment is the only way to gauge seriousness, and I think it would be good to talk it over with a professional familiar with kids, even if kid isn't taken to see said professional. It's way hard to get a real sense of it from what you have said, but I'd sure as hell rather err on the side of caution (without displaying overt hysteria).
If a young person knows someone who has killed themselves, or someone who has made a recent attempt, the risk rises a great deal.
Adolescence is a risk factor.
Boomer, if you wanted to email me with any details you don't want to be too public, feel free. Or not, of course!!!