Originally Posted by snakelegs
It's a good question, it's actually one that every biologist asks him/herself before conducting any study.
The only way to find out is to eliminate the possible forms of communication that are plausible, and if all of them are eliminated then you move on to other methods.
All communication can be broken down into simple steps
For sentient beings, there is a Sender who (considering the Receiver) chooses a message, encodes the message, chooses the channel for transmission, and the Receiver decodes the message. Any responses or feedback follows the same process with roles reversed.
For lower lever communication, one or processes may be predetermined; for example, instinctual responses to fear and anger result in physical and/or chemical changes that are transmited by a channel such as the smell of urea, the raising of hair, blood rushing to the skin, production of pheromones. In such cases, teh sender may not have a direct control over the selection of the message, encoding, or transmission.
How many different forms of communication can you think of?
Verbal/oral/Sounds/Vibrations - we can group these together
Emitting light, sign lianguage, dancing, demonstrations, etc... we can group these together
Anything which we cannot see or hear may be picked up in other ways so long as there is an energy signal distinct from background noise along probably sections of the electromagnetic spectrum. This goes back to the vibrations in the first section
So if those aren't being used, we have chemical production and reception.
Notice how none of these require any direct physical contact, but a proximity for another
If direct contact is required, then we can directly observe it.
When all else fails, we then move on to other systems.
All the best wishes,