I believe, to do a legally sound suicide assessment you need specific training on this. It's about the wording, how much you can really say about their risk, and demonstrating the effort you put into fully understanding their needs and risks. So, in short, a "legally sound" assessment will typically be longer than 30 minutes unless the person is just flagrantly suicidal (like if they present in the context of a suicide attempt). You need to do a thorough interview with the client, understanding their current mental status, risk and mitigating factors, previous behaviors/psychiatric history,etc. I like to use Phillip Resnick's suicide and risk assessment as a guide (it's a little outdated but still helpful
http://www.fmhac.net/Assets/Documents/2007/Resnick Suicide and Violence 2007.pdf). Then, I personally believe, you need to demonstrate effort made to substantiate the history, so calling providers and/or family, getting emergency records from previous hospitalizations, calling the area's crisis team for information on the client.
I would definitely read about it, but ultimately you need training, and if in the meantime this situation presents itself you can call a crisis team as they are trained to do this, and it relieves you of the burden. Also- nothing is ever "legally sound" when it comes to estimating risk, but you can CYA but putting in the effort.
And just for reference: I do these multiple times a week, and have for many years, so I think I've gotten the system down pretty well. I allow 3 hours per patient, but if the stars align, I can do it in 2. An hour to interview them, an hour for record review/collaterals (I typically already am given their records), and a quick dictation, And an hour to talk to insurance, if needed, and set up next level of care.