Goalkeeper Ruiter, 30, left FC Utrecht at the end of last season and has been training with the Wearsiders.
The Black Cats sold keeper Jordan Pickford to Everton for £25m, and Vito Mannone joined Reading for £2m.
"He's done himself no harm," Grayson told BBC Newcastle.
"But you can't make rash decisions on a player you've not seen too much in the flesh. As a manager I'd rather judge him in training rather than loads of saves in a game because that means he's being worked too much.
"In terms of his overall play, he was composed with his feet, very calm in possession and made a save when he had to. That's what you want from your keepers."
Sunderland trialist Robbin Ruiter impressed in Saturday's 3-2 friendly win against Bradford, but no decision has yet been made about a contract offer, says manager Simon Grayson.