How can one tell if a cutting has rooted?

I've left half-a-dozen cuttings for about 3 weeks
in a Lidl set of 6 covered and warmed containers.
The leaves all look healthy.
Should I transfer them to pots?

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Timothy Murphy
e-mail: gayleard /at/ eircom.net
tel: +353-86-2336090, +353-1-2842366
s-mail: School of Mathematics, Trinity College Dublin