I'm not aware of any internal "safety" device or mechanism aside from the manual crossbolt safety on Marlin leverguns. I have a .22, a .357, a .44, and a .30-30. They can, if dropped hard enough on the muzzle with a round chambered, fire. That'd take a major drop & a major impact.
They can fire if carried with a round chambered and the hammer fully down, if the gun's dropped in such a way as to have it land with the hammer striking a hard surface.
They can do the same if the chamber's loaded & the hammer's on half-cock, and the hammer lands hard enough to shear off the half-cock notch.
They can also fire a chambered round if enough force is transmitted to the hammer (either fully down or on half-cock) from any external source aside from being dropped.
The chances of this happening are very low, but it is possible, and not just with the Marlins. It would require substantial energy transfer and a rare combination of circumstances.
It's an individual choice, and I'm not saying what I do is necessarily what anybody else should do, but I carry my leverguns in all calibers for all purposes with a round chambered and the hammer on half-cock, with the safety off.
That leaves the gun in the most ready-to-go condition of all modes other than with hammer fully cocked, which I don't want to do. If I need a quick shot, I thumb the hammer back to full-cock as the rifle comes on shoulder, and I'm there.
Much faster & much quieter than cycling the lever as the gun comes up.
This offers an acceptable compromise between safety & speed for me.
An alternative would be to carry chambered & fully cocked with the safety on, but that doesn't appeal to me. Others do it, largely when hunting.
Denis