Well for most of them you can’t tell if legit or not.
Some may have a created at of today
Others will be stolen/hacked accounts.
And yeah most of them will appear to be “normal” names for fred_111 fred_123.
The people doing this do a lot of work to make the accounts look legit for the most part
Depending on what you use the data for will determine the mitigation if any that works best, but for the most part you’ll have to suffer the DOS from Twitch…
Twitch itself does peridoically do a clean up of such accounts.