When I use Smeargle's "Portrait" Poke-POWER to look at my opponent's hand, can I terminate the power's effect without choosing a Supporter card?
No, you cannot. You must choose a Supporter card when you use the effect if the opponent has one in their hand.
Source: Undaunted FAQ (2010-09-09), PUI Rules Team (2010-09-09)

Relevant cards

Note: All applicable cards may not be listed below.
COL1_21
HGSS3_8