for some species the only way to tell is to look at the coral after its dead.
price is usually a dead give away if its retail then you aint getting a colony of dendros for $40.....tubastrea no problems.
i currently have 3 types of dendros and about 10-12 different types of suncorals in my tank
some almost look identical :P
im not seeing a pic but maybe check out the nps forumn on reefcentral they have a good thread about the differences between dendrophyllia and tubastrea
cheers