Intent can be proven the same way intent is proven in a multitude of other criminal cases.
Doesn't the first step require just cause, to investigate for intent? Where are they going to get just cause to get any proof of intent? Are you arguing that getting pregnant gives them just cause? Are they going to stop every woman driving around and ask them where they are going, what they are doing, if they are pregnant? Are they going to go to the courts and ask for search warrants for their internet history and such based on the fact they are pregnant? Are they going to require all pregnancies to be reported to the authorities? Where is the just cause to investigate for intent?
It doesn't really matter because there is no crime in being pregnant or considering your legal options, which includes those out of state as long as those options are legal in that state.
Having the intent to do something that is legal in another state is not a crime. Having the intent of traveling outside of the state is not a crime. The whole foundation of such a law is based on legal activities outside the states jurisdiction and not illegal or criminal in that outside state. Your argument has no merit.
Seriously, step back and think. You are trying to argue about a imaginary law that is fully based on legal activities outside of that states jurasdiction.