MCS said:
I suggest wearing a Razorback (which is quite a common item, you should be able to find it on a drop) because if a Spy attempts to backstab you while you're wearing a Razorback, it will break, and you will be zoomed out if you are sniping, so you'll be able to react.
The Razorback is near useless, a Spy who even remotely knows what hes doing will pull out his gun and shoot you when you're occupied with sniping. Jarate is very good for detecting Spies, because if you know theres one around an area, you can splash him with it and trail him, even if he cloaks or uses Dead Ringer.