The broker should keep track for you.
I'm invested in several mutual funds and I left them alone after the original purchase. At the end of the year I had:
- some taxable dividends (got re-invested so I pay income tax but it increases my cost basis)
- some tax-free dividends (re-invested but does not increase the cost basis)
- some taxable short term capital gains (taxable now, but the new stock that was purchased has the higher cost basis)
- some unrealized capital gains (will be taxed if and when I cash out)
The broker will keep track and you will most likely get a combined 1099-INT/1099-DIV/1099-B at the end of the year.
