well, he kept on asking u if u were coming out on wed, and where u were going to take him 4 a drink. why would he ask u out for a drink if he wasnt interested? if it was as frinds, he'd have asked your mate to come too.
and then he tried to ask for your number, saying 'we'll have to keep in touch somehow'
and he kept looking and smiling.
i agree it would be hard if he said no, but if u say to him after work 'hey do u wana go for a quick drink?' if he isnt single, he'll say 'well ok but im meeting my GF/WIFE for dinner in a short while' or sth similar, then u still go, but just as mates. otherwise he'll say 'yea sure that will be really nice'