Not a silly question at all, as many people have asked the very same question before.
Practically speaking, you can tell the difference during the spawning season & possibly late into the pre-spawn season for the females, as they'd have bulging bellies from the developing egg sac. The males would be darker in their black pigment areas & those areas could even be covering more body surface than normal.
Technically speaking, there's supposed to be a difference in the shape of the anal opening ... round/oval it is male, tear drop/pear shaped it is female.
Many people ask this question so they can tell them apart during the pre-spawn / spawn period, with the intent of not keeping the females (so they can spawn) & just keeping males. Problem with that idea is that the males are the nest builders, egg fertilizers, and the egg & fry guardians. The more males you take from a spawning area, the less number of them the females have to mate with, plus they may already be in the process of guarding eggs (or fry) and removing them just puts a death sentence on the offspring. Luckily, it doesn't take a huge majority of the population having a successful spawn to replenish the numbers. Even though only a small percentage of each female's eggs will survive to become adults, the shear number of fish that are successfully spawned will normally be larger than the number caught or that die from old age. Weather & water conditions can interrupt or even prevent a spawn, altogether ... but, even that can be a minor bump in the road, and the Crappie will try again the following year.