Identify an acid from what?
The formula? Acids frequently start with "H", as in HCl, HNO3, H2SO4, H3PO4, or they may be organic acids with the -COOH group, as in HCOOH, CH3COOH, etc.
Some acids can exist as molecules outside of water. HF, HCl, and HBr can be gases, but when these gases are dissolved in water they dissolve and ionize to form the acid solutions. Look for the state symbol. HF(g) is molecular hydrogen fluoride gas, while HF(aq) is hydrofluoric acid since it is dissolved in water.
There is no "dihydrogen sulfate". The name of H2SO4 is sulfuric acid whether it is dissolved in water or not. That's because H2SO4 has acid properties regardless.
Water is a special case since it is formed in acid/base neutralization reactions. Water, HOH comes from the cation of the acid and the anion of the base, just the reverse of a "salt."