Hi there and not sure if you got your question already answered or even still need it to be answered :) What I seem to remember is that the M10 318 and 318 ('84-'87) has a chain, the M40 318 and 316 ('88-'91 & not many in the US) has a belt.

E36 = M50 = timing chain. E30 = M20 =timing belt

Although you did not specify the year, model, and engine size, here are some general guidelines:

All the 4 cylinder BMW 3-series (318i, 320i) had a timing chain.

From 1984 to 1991, the 6 cyl (325 series) had a timing belt.

From 1992 and up, the 6 cyl (323, 325, 328, 335 series) had a timing chain.

If the BMW you are inquiring about should have a timing belt, BMW recommends to change it every 60,000 miles.

Hope this helps a little.