A float test is used to distinguish the difference between nitrate and safety film bases. Sometimes there are no marks to indicate the difference between the two. One obvious difference is that nitrate burns quite rapidly and ferociously, whereas safety will only melt.
A float test uses the solvent trichloroethylene: a small sample of film is placed in the solvent — if it floats it is acetate, if it sinks then it is nitrate. It is important to ensure that the sample is immersed; otherwise it is possible for a nitrate sample to float on the surface due to surface tension.