Weeeelllllll....
You can test your pee - using ketostix (which I think are very unreliable, due to a number of factors)
You can test your breath - using a fancy gadget costing a lot (sorry, can't remember the name of it)
You can test your blood - using test strips like for bg monitoring (but you need a meter that does this, and the only one I know of off the top of my head is the Freestyle Neo, and the keto strips are something like £20+ for 10 of them )
or you can do what I do, which is 'feel it'
It takes a while to realise exactly what you are looking for, but after you have been very low carbing for a while, you get 'keto adapted'. More energy. More brain clarity. More ooomph.
Nowadays, I can actually feel the transition. I switch
from glucose burning:
'do I need a cup of tea enough to haul myself off this comfy sofa to go and get it?'
to ketosis:
'anyone want a cuppa? and while I am at it, I will organise tomorrow's packed lunch and just wipe down the hob...'