Good rule of thumb would be that whelks are found in temperate water and conchs are found in more tropical waters. Also, conchs have eyeballs, while whelks have eyespots. If you’re lucky enough to catch them while feasting, whelks are carnivores and conchs are herbivores. Also, their body colors are different. Conchs tend to be green or gray while whelks are tan or whitish (but can be other colors).