From type/Range

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method is-int(Range:D: --> Bool:D)

Returns True if both end points are Int values.

say ('a'..'d').is-int;                            # OUTPUT: «False␤» 
say (1..^5).is-int;                               # OUTPUT: «True␤» 
say (1.1..5.5).is-int;                            # OUTPUT: «False␤»