Trade fees are usually 0.3% of a total trade (we sometimes push it up to 0.5% for several weeks at a time to protect against front-running bots), but we'll explain here how to look up the exact amount.
Let's use this transaction here as an example, in which someone traded 4.4 sBTC into ~18,000 sUSD.
Fees are generated in XDR, the underlying Special Drawing Rights 'currency.' In that example, ~10 XDR was generated by the trade.
You can read the ExchangeRates contract here. To check the current XDR value against sUSD, query "rateForCurrency" with the 'XDR' symbol
The number it spits out is in sUSD (to 18 decimal places), and at the time of writing it spat out 5141597161322724050. This means the exchange rate was ~5 sUSD to 1 XDR, meaning that the ~10 XDR in the example transaction was ~50 sUSD, which is approximately how many fees were generated by the example trade.