。
She threw him a suspicious glance.
Literal
She [topic-は] him [to-に] doubting-looking [adj-な] glance [object-を] threw.
疑い深い ('suspicious, doubting,' literally 'doubt-deep') is an i-adjective; with ~そうな ('looking like') it becomes a prenominal modifier — 'glance that looks suspicious.' 一瞥 (いちべつ) is a literary noun for 'a glance,' more formal than just ちらりと見る. The phrase ~を投げる ('throw') is a metaphorical extension to glances and looks, parallel to English 'throw a glance.' The に on 彼 marks the target of the thrown glance.