Oh wow, congratulations Poshnickers (and Loz is right, that's a hilarious username) - what a great loss.
There's no magic way to tell if you're in ketosis. You can get wee-sticks which are supposed to tell you but they're not hugely reliable. You can sometimes get the metallic taste in your mouth but, again, I don't get that very often either.
Basically, to put it super non-scientifically: if you are sticking to plan, losing weight and not feeling rubbish then you are in ketosis. If you're in ketosis, you won't feel hungry and you'll actually feel fairly energised (unless you try to do something crazy like run for 10 miles) and your mind will clear and you won't have any other symptoms. You might also discover some slight changes to your habits: needing less sleep (I used to nap like woah but now 8 hours a night is more than enough for me) and TOTM regularising (but these could be general weight loss related, not ketosis related).
If you're pre-ketosis, you'll likely be low energy, tired, head-achey, fuzzy, grumpy and hungry!