They could never tell if Isabelle - Spoiler Alert.

Your right. No one could know with absolutely certainty without a note or some witness in the car or some other hard proof. However there were comments by the characters that it was determined to be the case so the police, forensic, medical, etc. investigations came to that conclusion and it was accepted by everyone.

Look at Australia. for years after the 1997 mass confiscation of guns they congratulated themselves for reducing suicide. In 2007 questions started to ask why there was a huge, immediate and sustained increase in self caused death by other means that could be suicide, but ruled accident after 1997, about a half dozen studies were done. In fact removing guns did not reduce suicide whatsoever, it just swept more under the rug:
<i>“These studies have shown that ABS has seriously been under reporting the number of suicides. The Queensland study reported that this under reporting had increased during the period under study and that the under count negates any apparent decrease in suicide deaths shown in ABS statistics.” </i>http://www.aph.GOV.AU/~/media/wopapub/senate/committee/clac_ctte/completed_inquiries/2008_10/suicide/submissions/sub42_pdf.ashx
<i>These findings confirm that routine mortality data have underestimated suicide mortality in Australia in recent years probably due to incomplete coroner data being available to ABS coders. Certain types of unintentional injury deaths have been over-estimated. Incomplete linkage of NCIS and ABS data in the source data used for this project complicates calculations of adjusted estimates and trends.

</i>Suicide decline in Australia: where did the cases go?http://www.researchgate.net/publication/24034093_Suicide_decline_in_Australia_where_did_the_cases_go
<i>“AUSTRALIA has dangerously miscalculated its suicide statistics - by as much as 30 per cent in NSW and Queensland - leaving a silent and growing epidemic of mounting deaths.
The figures are in stark contrast to years of backslapping by state and federal governments, congratulating themselves for reducing suicide rates from a peak of 2700 in 1997.
The Herald can reveal the suicide toll is as high now as it was in the 1990s - if not higher</i>
http://www.smh.COM.AU/lifestyle/diet-and-fitness/revealed-australias-suicide-epidemic-20090820-es3p.html