Method 1: Simulation using FEA tools, where you need to mesh the entire fabric and try to apply that force and check on how much movement it gives. This may need some good level of material property details of the fabric.

Method 2: As you know the kinetic energy is 1/2*Mass*velocity^2. Physically you can find out the energy observed by the fabric as difference in kinetic energy by knowing the initial velocity at which it hit the fabric and the final velocity at which it exited the fabric(if the bullet has enough penetrating power). Generally the initial velocity can be taken as the guns nominal bullet velocity. The final velocity can be measured physically. This may need some high speed camera or some sensing techniques, where the laser light source non availability time is the time at which it has cut the light source and travelled across(final velocity = bullet length / time cut - more accurate measuring electronics may be needed). Find the energy observed this way. If the bullet has not pierced, then the total energy was observed by the fabric (which means if there is sufficient space after the fabric, there won't be any energy transferred to the body next in line).

Generally you should measure projectile mass and velocity before and after impact.the difference in kinetic energy will be energy absorbed by the projectile.

How can i determine the velocity after impact when there is no complete penetration into the Fabric ?

If it has not completely penetrated into the fabric, it has lost all its energy. Hence the entire initial energy is absorbed by the fabric and the final velocity is zero.

The kinetic energy absorption by the target can be calculated as shown in the attached document.