You can find Mustard Family blossoms in the flower garden if you are growing Moonwort, Honesty, or Alyssum. You can often find mustard flowers in weedy places. Especially in the spring, in abandoned lots, along sidewalks, and maybe around the corner of your house you're bound to meet weedy Bitter Cress, Peppergrass, and Shepherd's Purse.
Since Mustard Family members usually, but not always, blossom in the spring, they tend to have white or yellow blossoms -- bright colors highly visible to pollinators in spring's relatively dim sunlight. More important to us identifiers, however, is the fact that Mustard Family flowers are structured in such a unique way that they can be identified at a glance, anyplace in the world.
Here's the secret, and if you look hard you can see what we're talking about in the flower picture above:
Mustard blossoms have four petals (very rarely none), and six stamens (the pollen-producing male parts emerging from the picture above). Moreover, of the six stamens, two are shorter than the other four.
If all plant families were so easy to define, plant identification would be a simple matter of memorizing "flower formulas." But, they're not. Nonetheless, it's good to run into such neat formulations from time to time...