You can request them to sit with you and use your laptop to browse through the codes and ask questions. Providing access to your code is a red flag.
Usually, corporate VCs sign NDAs. However, this happens often in M&A transactions where the acquirer insists on software audit (though typically conducted by a third party). If you have a working product/demo and clients, that should be enough validation for early stage. Be cautious and ask yourself, is this an investor I want to have on the cap table? Will they create problems for me down the road? If you are looking to take their money, I'd also interview the founders of their portfolio companies.