How will I know what my eighteen year old daughter keeps doing on mobile?

 I will tell you the way which does not require long and wide exercise!

Why don't you take precautionary steps instead of observing something?

At this age, various changes are taking place in the body of the girl, which no one knows more than the mother.

You also know that these physical changes create some curiosity in the daughter's mind.
She guesses what is the role of a certain part of the body? 

To know these curiosities, she can use the net or social media.

Now suppose she is doing something that you don't like!

So it is better that instead of keeping a CBI watch on him, explain to him how the body is maintained, and how it needs to be saved from decay.

If something is stopped forcefully, then the children will definitely do that secretly from which they have been stopped?

Tell him the advantages and disadvantages of avoiding friendship with the opposite sex (boys)!
Also tell him that it doesn't make sense to let someone else use your body! This is our body, 

so why should someone else trample it??

Don't consider the one who tramples as a boyfriend, 'hungry of lust'!!

The day young girls will understand this thing, there will be no need for such questions.

Hope you got the point!!

             Thanks