Create a precedent of "I always expect you to go through the whole combo." Even if you don't have anything, ask them to go through it. They'll never know if you have an answer or not. Now, this may force them to be a less greedy player by ALWAYS going for a counter to your answer, but they should probably be doing that anyway.
Also, I think your current strategy is fitting: you don't want to give the game away. Plenty of combo decks can have a 1% chance of not winning or the player can realize they needed 1 more mana, or 1 card that is their hand needs to be in the library or something--so it is quite reasonable. But I would say my way is less aggressive; "I don't I think you're a bad player, I just have this rule that I watch people go through their combo."
Also, I think your current strategy is fitting: you don't want to give the game away. Plenty of combo decks can have a 1% chance of not winning or the player can realize they needed 1 more mana, or 1 card that is their hand needs to be in the library or something--so it is quite reasonable. But I would say my way is less aggressive; "I don't I think you're a bad player, I just have this rule that I watch people go through their combo."
Modern
URGTemur ScapeshiftGRU
EDH
WGKarametra EnchantressGW
UBGSidisi, Brood Tyrant ReanimatorGBU
UBRKess DoomsdayRBU
WBGGhave TokensGBW
WUBZur RebelsBUW
WUBErtai CursesBUW
WRFiresong and Sunspeaker Spell SlingerRW