Hi Sonia
With regards to ketostix, you can purchase them if you want to be sure you are in ketosis, but you can normally tell by the strange taste in your mouth. You can buy ketostix from boots or any chemist.
On the putting it all back on again, what we are recommend is that you go onto the healthy plan to gradually re-introduce food, however if that isn't an option for you, then eating sensibly when you come off will not make you put it all back on again. It will take a little while for your metabolism to speed up again, but so long as you gradually up your calories, you will be fine.
The way I look at it is old eating habits = old size, new and healthy eating habits = slim for life!
So even if you have a big blow out on your birthday, you aren't going to suddenly regain it all. I can help you with introducing a healthy diet when you get to that point if you would like me to and the other boards on here have some great advice too. HTH