Often ordinary rubber firearm props are presented as being used in a very popular movie or tv series.
One way to help verify things is to check the model number of the rubber gun which indicates which real world firearm it was molded after.
For example with the AK-47 the Norinco MAK-90 is one of the most common AK variants present in the US. It was made in 1990. One can then see a rubber gun with this desgination could not have been used in a movie made before 1990.
Rubber guns claiming to be from various popular productions from sources such as Ellis Props or others can be checked with this method.
Taking it to the next level one could compare the actual serial number of the rubber gun to a list of serial numbers in use in a given year by a given manufacturer to further clarify when a rubber prop gun could have been used and when it could not have.