Is there an easy way to tell if this is a suit jacket or sportcoat? It's hard to tell by the picture, but the fabric is tweed. Even if it were a suit jacket, would it still be okay to wear as a separate given that it's tweed? It seems like that would be less offensive than wearing a generic charcoal suit jacket on its own.
There's no difference per se in a suit jacket or a sportcoat, the difference is in the fabric and occasionally the detailing (buttons, ect). Something made out of a fabric like tweed or a bold flannel is going to work fine as an odd jacket whether it originally had matching trousers or not. Something in a fine worsted charcoal would not.