Nipple holes slanted backward and forward are very rare. A few companies experimented with the concept at one time or another, but the idea never took hold because it introduced more headaches than it solved. IME, unless you have wooden or filled rims where the nipple is supported by the sides of a deep hole, odds are that the rim was drilled radially (except side to side).

Even if the rim is drilled forward and back you might still build radially. Use a hub or ring and 4 spokes to lace at 4 positions 90° apart and tighten slightly (not enough to overly flex the rim. Check to see if the nipples swing into the line of the spoke, or resist and set at an angle with a bend where the spoke enters. The former is fine to build radially, the latter argues against depending on degree.