As you know I've been out of Peru for a long time. But I am a water engineer.
For testing for bacteria you need a coliform test (ideally both for total and fecal coliform bacteria). Coliformd are an indicator of likely fecal and general bacteriological contamination. There are general guidelines on what is acceptable number of coliforms coming out of potable water taps.
The water company SEDEPAL will have testing kits but I'm not sure if they will help for a domestic tank. Likely you will need to look at getting the service of a laboratory or perhaps a water supply NGO who could use their kit to test your water for a donation to their NGO perhaps.
Found this looking for this sort of service: https://omegaperu.com.pe/servicios/
This is the sort of kit they would need to use: https://omegaperu.com.pe/producto/labor ... mo-humano/
It would be too expensive to buy yourself.
This is the main test for whether water is potable or not. If water is chlorinated adequately and not getting comtaminated with organic material before it gets to your tank you should have little to worry about (other than the taste of chlorine). Chlorine testing kits are much cheaper but retail ones I've used in the past in the UK have been a bit hit and miss. If you get a coliform test ask for a residual chlorine test for the water in your tank and going into the tank. Chlorine should also stop green stuff growing on the walls too which why it is used for swimming pools. You might want to redo the chlorine test after thoroughly cleaning out the tank as any muck in it will tend to absorb the chlorine.
Another test is a turbidity test which might tell you if the water has sediment even before getting to your tank.