Not sure if you always can. They can pick up worms pretty easily, as my cat did. I wormed her only when I recognised that she had worms. I did not do "regular" worming, as I didn't like medicating my cat.

Same with fleas, only "deflead" her when she showed signs and eventually it became less and less anyway and I barely had to treat her for either.

I would see the worms coming out of her backside (excuse this, I know it's gross), so that was obvious! Also they can get a pot belly sometimes and/or show signs of being underweight, even if they are eating excessively. Maybe show signs of itching and try and rub their backside on the floor etc (though dogs normally do that).

My cat did excellent on raw meat by the way. I found canned catfood tended to not do so well with her. She did excellent on beef and things like that. Lamb, chicken livers, etc.