What is the most appropriate way to correct a Senior NCO?

SSgt (Join to see) just approach the SNCO and say something like...

Excuse me SMSgt Snuffy just wanted to let you know that your trouser leg came unbloused.

He/she should reply something like...

Thank you Airmen Williams I appreciate that.

Every rank in the military has the general military authority to make an on the spot correction and if the SNCO or officer is a good leader they will appreciate the tactful correction. I hate being all chewed up and nobody will make a correction because they are afraid of repercussions. Nobody is perfect we all need a little help sometimes.

Always check yourself first, before approaching someone who outranks you. Don't correct him or her in front of their own peers, it might embarrass them. Approach them and ask for a private word with them. They should take your correction the wrong way, you're after all, making them look better after a slight lapse, perhaps a bootlace fell out, or a charm on a necklace is hanging out (that's my downfall).
Good luck and I'm sure you'll do just fine. Oh yeah, don't be nervous, SNCOs are people just like you. They've just been around longer, no big deal.

CSM (Join to see) gave an excellent example, but if it's a correction you need to bring to their attention in a private manner, something like : "Rank Name, could I speak with you offline for a moment" and direct them elsewhere, once there inform them of the correction and if necessary something like "I asked you aside as I didn't want to bring it to everyone's attention". Has worked well in the past for me.