Yes if there's to many.. it dont matter one flea is to many. If the cat is real young dawn dish soap (regular) will do wonders. If the cat is older than 6 mo go get some frontline, or go to your local vet and request flea drops... they carry the most effective and will be not much more then what you would pay for a good brand in the store.
And as far as symptoms.... itching for starters lol, but is the cat has them really really bad they will get anxiety, lose weight, become lethargic, diarrhea, puking and so forth....
Either way one flea is to many... and if your cats sick and you have no clue as to why... try calling the vet and describing their syptoms.