You have put a highly philosophical question. I do not know to what extent I can clarify your doubt through this mail.
The very ‘existence’ is ‘Truth’. Truth does not need a proof. It is a proof by itself. In the higher realm, discrimination between the real and the unreal is the method. Real is Truth. That which stands in all the 3 times (past, present, and future) is truth.
In the day to day life (Vyavaharika) Truth refers to certain statement of facts to our best of knowledge. Supposing a glass of milk is placed before you mixed with poison. I openly tell you about the fact and request you to drink the milk. Will you not use your discrimination and reject it because it is harmful and dangerous to your health and life? Sticking to the Truth gives us moral strength to face the life as it comes, though at times the results may not be encouraging in the beginning. But the end is ‘strength’ and ‘Joy’.
Though it is not advisable to tell even a ‘fib’, the unpleasant truth should be avoided. These is a Sanskrit Shloka which says: Satyam bruyat priyam bruyat, Na bruyat satayampriyam

