Training is need based and some responsible manager must have felt such a need to have some person / persons trained on some particular gap.
Once that particular training is delivered, that responsible manager can easily evaluate the trained person / persons and if an improvement is realized, the given training is effective.
So even if the manager reports something like 4 out of the 5 people trained have demonstrated improved performance over some reasonable time after the training, it is sufficient as a good training effectiveness monitoring.
The problem starts when a gap is not identified and some training is provided (say like 15 persons attend an in-house session on some topic because their assembly line was down due to shortage of material) for which effective is now sought after .....