How to determine whether plastic is ABS or Polypropylene?

It all depends what are the tools you dispose.
Infrared analysis will tell you easily whether it is ABS or PP. If the material is too thick for a transmission measurement, you can always use ATR (Attenuated total reflectance), which allows you to measure some microns inside of the solid. Raman would do the trick as well. For the three methods, the CH and CH2 group spectra will only appear in PP. On the contrary, ABS will show aromatic rings, which are active at different wavebands which allow to see the difference.

Thermal analysis (e.g. differential scanning calorimetry) would also be useful; it is destructive but you only need a tiny piece. As ABS and PP have different glass transition temperature and melting point, it is possible to distinguish them.

There is often a non-critical location on the plastic extrusion that you can test some of the solvent on. Rubbing a little with a paper towel you can see discoloring and if any of the plastic is removed. Acetone will detect ABS really well if your specific solvent is more slow acting. The blemish is usually just a dull area in the surface.

Remember that even though the plastic housing may survive the solvent, your bearings and seals may not. Another consideration is flammability and the likeliness of a failed pump catching fire. You may want to do a search for pumps specifically designed for your application or highly chemically inert pumps like santoprene diaphragm pumps or silicone tube peristaltic pumps.

ABS has an acrid smell when heating is applied. The flame is yellow with blue edges and will not drip. ABS will continue to burn when the flame source is removed and the odor continues to be acrid, the flame color is the same yellow with blue edges and the material will drip. The rate of burning is slow and black smoke with soot is admitted into the air.

Polypropylene smells acrid when a flame is applied and the flame is yellow in color. It will continue to burn after the flame source is removed giving off a sweet smell. The flame at this point is blue with a yellow tip. PP produces drips as it continues to burn slowly. When bent, polypropylene sheet will turn white on the crease line. Interestingly, PP floats in water (density < 1.0 g/cm3).

ABS and Polypropylene have very different chemical resistances, with Polypropylene very resistance and ABS no so much. Acetone, Methyl Ethyl Ketone (MEK), 2-Butanone, and Methylene Chloride will dissolve ABS but not Polypropylene. Thus put a small drop of the solvent on the surface, and mush it around a little with the tip of a knife. If you see some plastic softening and diluting, it's ABS.