The plants can be identified but only definately when they have produced a flower, the shape of the stem etc all contribute to the identification. However in the beginning you can watch the plant as it grows, it will be either a bush type or a vining type, once that has been identified it may be fairly easy to guess what yours is. If it is a courgette/marrow it will be a bush type if it grows into vine and starts to grow long it will not be a courgette/marrow, it may then be a pumpkin but it also may be some type of winter squash that vines too.
How big is your plant now?
I can help you further if you give me some more info.
XX Jeannine