My 18" wingspan RC plane now refuses to takeoff. Used to fly well before, then the prop fell off, replaced it with new prop but now just drops. How to determine if it is the right prop or something else? It has a rear prop,

Do you have the propeller facing in the right direction?

Yes, it blows air rearward

Is your new prop the same size and pitch as the old one? Look for numbers on the front face that reads something like, "8x4". The 8 means a 8" diameter prop, and the 4 is the pitch, which means the prop theoretically moves forward 4" with every revolution.

If the props have different numbers, then you need to get a new prop with the same numbers as the original prop.

If it's a factory made plane it should have came with a manual or you could probably find it on line. The propeller size would be in the manual.

Lost track of what is the correct prop for this f16 plane, my present prop is 2", is there any correct prop spec for this plane?

Contact the manufacturer for where to buy spare parts. They may have a web site with an on-line manual.
You gave us no information about who makes it, what model it is, how old it is, etc. - so no way we can know what to recommend.

Just a note. A backwards mounted prop will still blow air in the correct direction but with much less efficiency. Mounting a prop backwards is an old free flight trick to get reduced thrust for test flights.

In other words you need a PUSHER PROP of proper size, a regular prop wont hack it.