Is there an anti siphon device in the gas tank of a 95 Ford F-250 with dual tanks?

No, Ford F-250 trucks from 1995 (and most other vehicles of that era) did not typically come equipped with anti-siphon devices in the fuel tanks themselves. Anti-siphon devices were and are much more commonly found as aftermarket additions or in specific situations dictated by regulation (like for certain fleet vehicles). The primary theft deterrent relied on locking fuel caps.