Always_Ask_Questions,
“nobodu would ask whether someone us frum when you see him eating Mcdonalds burger, but somehow money aveiros are not that clear cut”
Nobody can really know what is in another’s heart, but we typically call someone who is specifically shomer Shabbos and Y”T, kashrus, and taharas hamishpacha “frum”, because in most cases a fear of Hashem and belief in His Torah are the only reasons for adhering to such a disciplined way of life. It’s not a perfect demarcation obviously.
Also, nobody is above mistakes and falls, so being frum does not mean a person is free of sin. Someone who lies or steals habitually, however, demonstrates that he has no real trust in Hashem as a provider, and no respect for His Torah.