I connected right to my secure network

I just set up my new router. I had it set up secure, with WPA-Personal ( PSK ), not to broadcast my SSID, all the things I knew to do.

I went to set up my internet radio, and just as I set it, I had to manually put in my network.

I then wanted to set up my laptop. I booted up the laptop and it connected to my "secure" network automatically, without me having to enter anything.
I didn't' have to enter a password or anything. This is a totally new, internet and router, laptop has never been on it.

If I can do it, how do I know if someone else can connect even tho it's "secure? "

Thank you,
Chris.