Hi Xphile
You can't really tell if an ancestor was Romany from census returns. Mostly people are aware of a Romany background due to little snippets of information passed down through the family.
However, indications (from census returns) that a family might come from travelling stock are:
Frequent movement ie. children born in different locations
Persistant itinerant occupations/descriptions ie. Gypsy, traveller, stranger, wayfarer, hawker, scavenger, tramp, peg maker, basket maker, chair bottomer, besom/broom maker, dealer, marine store dealer, mat maker, tinsmith, whitesmith, pedlar, fair keeper (many Gypsies worked the fairs in those days as well as the more proper showmen) etc. etc.. Obviously there are people following these occupations that didn't have traveller origins but they are some of the signs to look out for.
Abode given as cart, house cart, van, caravan, tent, shed, barn in the address column. Prior to 1861 there was no requirement to list those not living in houses but in 1861 special forms (usually at the end of each district) are to be found recording those not living in houses. In the later census, these families are mostly listed in chronological order as they appear on the census enumerator's route. Although there was no requirement to list them in 1841 and 1851,some counties have better coverage than others ie. Sussex has quite a few whereas Kent is not so good.
Best wishes
Casalguidi