Do they really check how many months you have stayed in Europe when you get on the train? Because apparently you can't use Interrail if you haven't stayed for at least 6 months in Europe before your trip.
When purchasing an Inter Rail ticket you have to supply the relevant documentation to meet qualifying requirements. In my case it was a valid Passport which showed I am a citizen of a qualifying country.
I doubt you will be able to 'get round' these requirements. And I doubt you will want to be on the end of getting found out either as that is fraud.
If you live outside of the qualifying countries you can apply for a Eurail pass instead.
Your question is explicitly about BOARDING...
They check your entitlement when you apply to get a pass. When boarding, they may check your travel report. If the travel report does not flush, they will be entitled to examine the supporting docs, and if those do not flush, you will be detained for fraud. They do not always check, but they will always prosecute for fraud or abuse. Each and every time..
So the answer to your question is 'yes', they really check, but not your eligibility INITIALLY; and not in each case. And the secondary answer is that it's virtually impossible to be in that situation without committing fraud on the application, and they have no statutory leeway to do anything other than detain you and turn you over to the authorities. They have no other option.
For what it's worth: They are also entitled to check when you DE-BOARD...