For the best view on the fields, take a tour.
As the tour drivers check on the fields and keep each other updated, you have the best chance of seeing the best views of the fields.
Most fields are 'harvested' as soon as all flowers are open, as the flowers are cut off to improve the growth of the bulbs to get a better flower next year.
That means that most fields are only in flower for a few days, so it is hard to get a good view with many fields in flower by random choice or by accident.
There are flower fields along most of the western coast of the Netherlands north of the Hague, and there is a train line running through most of that area, but not all fields are convenient close to the railway line.
The spring flowers bloom, depending on the kind and the weather, from February to June, with April as most likely to see a good selection of fields.
In the rest of the year you might see other flowers. In the same areas there are growers who develop new variations or who just propagate for selling the plants, and for cut flowers. In some cases the flower stems will be cut before the flowers open until there are too many coming to the market and the cost of harvesting is bigger than the money they sell for. This makes for unexpected fields full of flowers. So any time you are traveling in the Netherlands you should keep your eyes open, as you might see 'fields full of flowers'.