When windows technician asked me to insert mosquito net on the office toilet, I felt worried if someone inside the toilet. So, I knocked the guy in the next room to check if its vacant or not. When I knocked and open her room, she said: please coming. Then I answered her like this sentence:
I just want to check if you are inside the toilet or not because windows technician want to insert mosquito net.
There are several mistakes in you sentence. It would be better to say “I just wanted to check if you are inside the toilet or not, because the windows technician wants to insert a mosquito net” (or " insert mosquito netting"). “install” would probably be a better choice than “insert.”
In American usage “inside the toilet” would mean that the person was actually inside the white ceramic fixture and not the room. Bathroom, rest room, ladies room, or washroom would be used