How can I tell if my dog has a thyroid condition?

He is a jack russell mix, and he is probably 50 pounds. He gets one cup of food and one at night, we do feed him some human food but it is not an extreme ammount. He usually will just catch some spilled cereal on the floor rarely or we will give him a handful of popcorn, so I don’t think us feeding him our food is the main cause of his obeseness. Should i take him to the vet to find out?