How do I know it’s an allergy?

[chargedproton]

DS is 8 months old.

He’s just got over Gastro. Been okay since last Thursday.

However today he’s been throwing up his feeds again and has a cough, otherwise he’s fine (his bottom teeth are coming through as well).

Yesterday he had banana, Greek yoghurt and chicken.

He’s had small amount of banana before so don’t think it’s that.

I introduced Greek yoghurt to him about two weeks ago and he had Gastro the next day. Same with chicken.

He’s formula fed and has had natural yogurt before but I think it’s the Greek yoghurt?

How can I be sure or eliminate? I don’t want to do any damage.