If you know you had chicken pox as a child, you're immune. The problem comes in if you didn't have chicken pox as a child, OR if you got the vaccine which doesn't protect you for life. If you had chicken pox as a child, you're protected for life.
Not always true...
My child's Dr said that since my children we're both given their shots for pox, that they will never... NEVER get it.
I freaked out cause my 6 yo daughter had a rash so I took her to the Dr thinking that's what is was. (It just just a great rash) but she explained they will never get it.
Also if you never caught it as a child you may already be immune to it. My mum said when I was younger she used to send me to other children's houses who had chicken pox and I never caught it. I've still never had it and my doctor said it's unlikely I will but I'm more at risk of something called shingles when I'm older.
You can request a titer- it's a test that will tell you if you've ever been exposed and have the antibodies already in your body to fight it. I never had it and never got immunized- got the titer and found out I've already been exposed in the past and am immune now.