java - testing - lambda with inner private method?

I have some problems with unit testing the following method, I'm quite new in this business and it would be greate if someone could help me.

    public List<GetSupplyChainResponse> getSupplyChains(){

    List<GetSupplyChainsResponse> response = new ArrayList<>();

    supplyChainRepository.findSupplyChainsWithCompound().forEach(result
            -> response.add(getGetSupplyChainSimpleResponse(result)));

    return response;}

getGetSupplyChainSimpleResponse() is a private method of the same class as getSupplyChains()

Is there any possibility to define return values therefore or do you have any other ideas how I could test the method getSupplyChains?

Thank you so much in advance!