To express a counterfactual conditional the same non-manuals are used as in factual conditionals. These include raised eyebrows, head movements and other non-manuals as body lean and eye gaze [Syntax 3.5.1.1].
Facial expressions, such as squint and the mouth gesture with corners down (c-down), indicate the counterfactual conditionality as well. On the one hand, the antecedent clause can be marked by squint combined with raised eyebrows and a head nod, on the other hand, it can be marked by the mouth gesture c-down combined with raised brows. In both cases the manual sign if1 or if2 can appear additionally.
hn
re
sq
a. if1
b. mouth gesture