Tudor Jones, 25, and Gill, 29, have barely featured for the Canaries since signing 12 months ago, but Askou made 24 appearances last season until his campaign was cut short by a foot injury.
Roberts, who scored 97 goals during a seven year City career, believes the Germany trip will help Lambert to decide who will be in his first-team for the season-opener with Watford on 6 August.
"When I was here I went to Scotland two or three times and I've been to Sweden and Austria," he said.
"Paul Lambert played in Germany. He knows it will put them in good stead and will give him some sort of idea about where his team are and how hard they need to work from now until the end of the season."
The squad arrived in Germany on Sunday and are set to train there for a week, returning in time for a friendly at Stevenage on 20 July.
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