You won't be able to 'tell' if a plant is aquatic or not just by looking at it until you get some familiarity with true aquatics vs. non-aquatics. Lots of stores will sell you a plant that is non-aquatic and soon it will start to die off and rot in your tank causing water quality issues along with looking nasty. If you have anon-reliable lfs, your best bet is to look online for good aquatic plants that will work with your lighting/fertilizer/fish, and taking images to your lfs to compare; or, the reverse, go to your lfs to see what they have in stock, and take pics and write down the true latin name of the plants if your lfs knows, and then compare online to ensure they are true aquatic plants.