how to test isolation of DCDC convertor?
I assume that you have from 1 kV to 2 kV available as required for testing. If your question includes "how do I generate 1 kV" or similar please make this clar in the question.

All available data and a photo would help.

Non destructive testing may be difficult.

If you insert only the components that are relevant - eg those that cross the isolation barrier, and then apply a high voltage via  suitably high resistor and monitor current, you can get a reasonably good idea of whether you have leakage.

As a system rated at 1.5 kV should protect against somewhat more than the rated limit you can do a pass/die test with 1 kV. As above you can feed the HV via a high value resistor to limit current flow if breakdown occurs. Breakdown does not have to be destructive to anything on the isolated side BUT it's very easy when you are applying 1.5 kV to have a proportion of it turn up where it shouldn't.

IF the part is from a reputable manufacturer then you are really testing your PCB. Soldering in a dummy device so that solder etc is the same in real and dummy systems and then cuttin the dummy away so only its leads are left will give you a PCB that closely resembles your finished product.