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One way to test is to take it out of the water. If it still stands up, it is probably not a true aquatic. You can always watch it closely and see how well it does. If it starts to look bad, toss it.
There are quite a few true aquatics that will stand up just fine without water (P. stellata "Broad-leaf" comes to mind) so I don't think that is a good test of submersed growth fitness. The only real way to tell, if you can't identify the plant, is to watch it for signs of deterioration. Keep your eye on it fishygurl, and if it goes downhill, pull it out. It is a cool looking plant though