If its a blown baffle (the partition between chambers) then air would leak from one chamber into the adjacent chamber but the raft would stay tight once you pumped it up. Test for this by inflating one chamber and then the chamber on the opposite corner (for example, the front right and back left chambers) of the raft so the chambers between them are not yet inflated. If the chambers between become inflated, then yes, you do have a blown baffle and its time to seek professional help.
If the raft is losing air and going flat, then you could either have a leaky valve, or a hole in the raft. inflate the boat so its tight and if you can't hear that depressing sound, then air the boat up and spray it down with soapy water like Kimmy suggests. When you see bubbles, that's where the leak is. Then use the rest of the soapy water to clean the dirt off the raft, rinse, let it dry and start patching it using glues and material appropriate for the raft material (likely PVC or Hypalon). There are a number of good raft repair companies in the Front Range, such as Raftfix.com or Downriver Equipment if you're not ready to do the job yourself.
Nothing in the world is more yielding and gentle than water. Yet it has no equal for conquering the resistant and tough. The flexible can overcome the unbending; the soft can overcome the hard. - Lao Tse