How to verify purchased votes

During elections, candidates who engage in vote buying to buy their way into office, have to implement double-checking measures to make sure that the voters they "buy" really voted in accordance with the candidates' dictates.
In other words, if a candidate buys the votes of Juan, the candidate should make sure that Juan really voted for the candidate and his pasrtymates.
How does the candidate make this verification?
There have been many ways to do this, but there is a new tactic.
The new tactic is to use the "coding" method.
Its like the color-coding scheme of vehicles in Metro Manila.
Candidiate X will approach a family in the barrio.
Candidate X and / or his agents will bring the cash and show guns, just to intimidate.
Candidate X will tell one family in the barrio, say a family of six, to place a "code" in the ballot.
This code cannot be detected by the board of election inspectors.
And this is the ingenuity.
Candidate X will instruct the family of voters to fill the last slot for, say councilors, board members or senators with a pre-assigned code.
The code can be the name of a person like "Pedro."
So for that entire family of six, they must place the codename "Pedro" at the last slot.
So when the votes are tabulated during th canvassing, the watchers of Candidate X will identify those six ballots which have the code "Pedro"
If there are less than six who entered this code, then the family should beware for candidate x will get back at them.
Each group of voters or each family have their owned pre-assigned code to be placed in the ballot to identify their ballots.
This kind of vote-buying verification is effective and will not be detected by the election inspectors because the the codenames will just be considered invalid votes, but actually, it was the signal for the corrupt candidate that indeed, the deal was "successful."