Demo won't show you any of this — demo fills are kind by design (English, hope that's ok). What I do: open a minimum-size live account, run a logging EA for 2-4 weeks. It snapshots spread every minute around red-folder news, logs requested vs filled price on every order, and execution time in milliseconds.
Run it on 2-3 brokers in parallel. The test costs you a few small trades' worth of spread — far cheaper than learning the same lesson with real size. I posted my own 30-day numbers in the Brokers category last week if you want a format to copy.