How can I tell if an Implant is loose?

you need to see your dentist asap.... to check the implant Depends what is loose ...
If the screw over the implant is loose, i can be tightened and it should be tightened as soon as possible before damaging the actual dental implant
If the dental implant itself is loose, then it should be removed.  A loose implant is a failed implant and should be removed before further infection and bone loss ...

Go back to your dentist and have them check the area again. It may just be the cover screw or healing abutment (not the implant) that is coming loose. This is often an easy fix. If the actual implant is loose or failing, then it will probably need to be removed, allowed to heal and the procedure performed again. 

I suggest calling your dentist if you are having a problem. Most dentists have an emergency number for the holidays. Speaking to them directly would be the best way to get your question answered.

the implants are tightened with a screw , so due to masticatory forces, it might got loose, so just go back and they will tight it again.this is normal, when you have not screw them in with the torchimeter at 25 newtons.