Having toured with the Lions as a player in 1997 and 2005 and having spent more than a decade-and-a-half at the coal face of top-flight rugby with Leicester Tigers, Rowntree is the perfect man to assess who could make an impression against the Springboks.
Now employed as England scrum coach following a stint as an assistant coach with Leicester, Rowntree has been working closely with fellow 2005 Lions tourist Andrew Sheridan.
Rowntree admits he was impressed with Sheridan's performance against Wales a fortnight ago and is hoping for more of the same against the Irish in Dublin.
"Andrew has come along in the last few weeks," said Rowntree.
"His work rate is colossal and I don't use that word lightly, along with the rest of our front five.
"He will be judged because of the man he is and the size he is with what he does in the scrum. That is developing nicely as well and another increase in performance and we will be happy at the weekend."
With Sheridan and England having already faced an impressive Welsh front row in Cardiff, Rowntree is expecting an equally-tough test against an Ireland front row packed with Lions hopefuls.
"Against Wales it was a straight Lions trial with Andrew and Gethin Jenkins," added Rowntree.
"The Irish front row of Marcus Horan, Jerry Flannery and John Hayes - the Munster outfit - are a handful.
"They are all improved players. I am a big fan of Flannery and they are all contenders for that Lions trip."