How to know if a zip lock bag is safe for sous-vide (or boiling), without indication from the manufacturer?
You don't mention the brand you use but, as many of us do, refer generically to ziplock bags. For the brand name product, SC Johnson, which makes both Ziploc® brand bags and Saran ™ Wrap, states that it does not use BPA in these products. 


  SC Johnson’s Ziploc® brand Bags and Containers are BPA free. Our products are extensively evaluated for toxicity and safety and comply with applicable quality and safety regulations.


If you brand you use doesn't make a similar statement on the box or the manufacturer's web site, then better to use Ziploc, not an off brand.  

Modern Cuisine noted that: 


  [T]he safest plastics for use with food are high-density polyethylene, low-density polyethylene, and polypropylene. Virtually all sous vide bags are made from these plastics, as are most brand-name food storage bags and plastic wraps such as Saran Wrap.