This information can be found on the issuer’s web site. It is usually in a document that can be found from a link on the ETF’s main page. It is needed for tax reporting. Eg. iShares EMIM, an accumulating ETF. We look at the Literature tab, and then find a document listed called
iShares PLC Reportable Income 2019 - tax information
Download this spreadsheet and find the ETF. This one is on line 68. We read there that the dividend was $0.5264 per share, deemed distributed 31 December, 2019. This is the amount that was reinvested for you. For a UK taxpayer this amount is taxable. As this is USD you have to do the work of converting this to GBP using the exchange rate of 31/12/19.
As you can see, it requires some work. I don’t know of any broker who will do the work for you. The problem is that there are too many ETFs and people have different tax reporting requirements. One way to avoid the hassle is to simply make sure you do not own the ETF on 31/12. Sell it just before and then rebuy, or hold accumulating ETFs in an ISA. Now for further bad news: even for distributing ETFs you still need to do this work - if you are a taxpayer. That is because distributing ETFs, even though they distribute most of the income, there is a small bit that is retained in the fund and which is also considered “excess reportable income” (ERI) and is taxable.
For example, IUSA is a distributing ETF, which makes 4 payments each year. But if also had $0.0008 per share ERI in 2019. (line 80 in the spreadsheet)
Accumulating ETFs deem the dividend to be reinvested at one date each year, although in practice it is happening continuously.