Unfortunately, as with most illnesses chins will mask their symptoms. Weight loss, a change in eating habits, and of course, change in poo consistency are all early warning signs. The mucous covering the poo may also be an indicator, but I am not entirely certain of that. With Giardia, once the animal has reached a full infection they will have all-out diahrrea, not just soft poop. However, softer poo can be an early indicator, as the symptoms escalate as the parasite multiplies. Of course, one of the major concerns with diahrrea is that the chin can become very dehydrated, very quickly. A dehydrated chin will be lethargic and will not eat. By this time however, you have on sick chinnie on your hands.
The basic rule of thumb is this: you know your animal the best. If you think something may be wrong, go with your gut and calll/take it to the vet for a checkup. Over time, you will be able to learn what constitutes a dire emergency and what you can maybe treat yourself.