From collocation, meaning, personality of the speaker, etc, i.e., from the context, as always. For instance, 知らぬ間には与えられん is always negative. (This is spoken language, not written language.) Furthermore, the writer/speaker would use ぬ or ず for negative and む for presumptive if it's really confusing.
In addition to ん for presumptive, this auxiliary verb also can express the speaker's will when used with volitional verbs, so 蘇らせん can mean 蘇らそう "to be going to revive ~". (It's similar to the auxiliary verb "will" in English, right?)