Limpet refer to many kinds of snails both marine & freshwater which have a low conical shell with big opening at the bottom. They will cling to surface tightly when disturbed.
A true limpet - Patelloida sacharinoides
How do we distinguish which one is a ‘True Limpet’? True limpets usually have fewer ribs on their shell compare to non-true limpets. Non-true limpets usually have ribs pattern that are symmetrical. If you found a limpet shell, just look at the underside, if you can see a canal, chances are they are not true limpets. 
Siphonaria atra - not a true limpet
Underside of Siphonaria atra
Look for empty shell on the shore and never try to turn over or dislodge a limpet by using brute force. Doing so will destroy or kill them as limpets will rather destroyed than let go their grip.
