Intel started locking the multiplyer with the Pentium 133 (pre mmx). Not all the 133s were locked but that was the start. They also did not lock all their production at the time because many of the faster chips, such as the 150, were not locked.
It is simple to determine if it is locked or not. Just try running it at a faster multiplier by making the appropriate jumper change on the mobo or using the cmos setup (if that feature is included). If it runs at the expected speed then the multiplyer is not locked. If it fails to boot or starts at some strange slower speed, it is locked. You will not hurt anything trying this,