You won't always see the fleas, but the dog would be itching a lot and you would probably see little red bite marks from the fleas and possibly flea dirt. The best test it to use a flea comb or nit comb and put any offending small blackish coloured 'grains' on a piece of white paper. Then drip a couple of drops of water on the 'grains' and see if they turn red or pink, if they do it is probably flea dirt.
If s/he does have fleas remember to treat all the bedding and areas of the house s/he is allowed in as they may also be living in the carpets and furniture.