This will require ongoing accompaniment and follow up with those trained who should be asked to highlight what use they have made of their training.
Of the activists that have been trained, follow up a random selection of at least 15 in each category (age group, gender and other identity marker relevant to the project such as faith group or ethnic affiliation) to identify the proportion that have applied their training. You can then take this as an indication of the overall number that remain active. So if you find that 4 of the 15 women and 7 of the 15 men trained that you followed up with went on to train others then you can assume that 37% of those trained ((4+7)/(15+15)) are active champions