1) Can you give someone food if you’re not sure whether he will say a bracha?
2) If someone asks your advice and you really dont know but you tell him what to do and it ends up being bad was it lifnei iver?
3) Is selling cigarettes lifnei iver?
1) It is permitted to offer food even if the person might not make a berachah.
2) Giving bad advice is included in the prohibition of lifnei iver. See Rambam, Rotze’ach 12:14. In the specific case, “bad advice” was given where it would have been correct to declare ignorance, so that it appears the prohibition was transgressed.
3) It appears that selling cigarettes will be lifnei iver if there is nowhere else to buy them. If there are other places to buy them, it will not be considered lifnei iver — however, there are opinions who write that this is true only where the alternative is by non-Jews. See here.
Note that the Magen Avraham (6) writes that giving food for tzedakah is permitted only when the person in question might not make a blessing, yet cites from the Eliyah Rabba that even when he will certainly not recite a berachah it remains permitted to offer the food, provided that his not reciting the blessing derives not from wickedness, but from ignorance (see Mishnah Berurah 11). This is usually the case for secular Jews.
See also Shevet Halevi (8:17, permitting giving food when the negative reaction of not doing so might cause worse transgressions), and Teshuvos Vehanhagos (1:483, permitting inviting a secular Jew for the purpose of bringing him closer to Judaism, but limiting this to inviting him for a meal, and not to offering something extra).