What should I look for in a foundation or makeup to know that it won’t make my acne worse?

"Non-comedogenic" Look for the term "non-comedogenic" -- meaning that the product lacks ingredients known to worsen acne in most people. Key word being "most" people, as no product can have a 100% guarantee in all people. Avoid heavy & oil-containing products (including your moisturizer & other face products), as these tend to worsen acne.
Makeup for breakouts. I advise patients with acne to go with a mineral based makeup line. My favorite is jane iredale. Mineral makeup is generally lighter and healthier. Jane is pure pigment, so there is no filler or talc in there to clog the pores. The minerals, such as zinc, are calming and anti inflammatory. Jane is bacteriostatic as well, meaning it makes your skin a hostile place for bacteria to grow.