I think that there's no such thing as "complete assurance"
Kahit gaano katagal, gaano ka kaganda, gaano ka kabait, you can't ever really tell.
The most we can get is reasonable assurance that the partner will not cheat. Maski tayo, honestly, we can't really say we're sure that we won't cheat, unless we're faced with a very 'challenging' situation.
All that we can really do is to focus on continuously working on the relationship and constantly ask ourselves if we're still the kind of person na masaya at masarap kasama. That's the only thing within our control. And even that is not a guarantee.