I would like to know if choked flow can occur in a valve even if the velocity of air passing through the valve has not reached Mach 1.
Also how will I determine is the flow is choked or not, given I know the upstream and downstream pressures.

Take a look to the Venturi's effect: it could be useful.
If downstream pressure is less than

P(crit)=P(upstream)*(2/(k+1))^(k/(k-1)

Then the flow is critical and velocity is 1.0 M.  For air this works out to 0.528 times upstream pressure (in absolute units).  For typical natural gas (i.e., k=1.28) it is 0.549 times upstream pressure.

If the air is at 200 psig (NEVER say "200 psi" on eng-tips.com, it means nothing and makes it harder for people to help you) and you are at sea level then P(up) = 214.7 psia and P(crit)=113.36 psia=98.66 psig.  As long as the exhaust pressure is less than 98.666 psig then you will have critical flow and velocity will be 1.0 M.

Now if the values you gave were psia then P(crit)=105.6 psia=90.9 psig so with 100 psia downstream you are choked and velocity =1.0M

Choked does not mean that the flow stops.
Choked does not mean that you can't get any additional flow through the valve.  Just open the valve(Balance valves) more and you'll have more flow.  
It really only means that the actual capacity is less than that predicted by the noncritical Cv equation.  Once the deviation of the measured flow less than the predicted flow exceeds 2% the flow is said to have choked.  
Another thing that happens is if the flow is choked and tnothing else changes, it is possible to lower the downstream pressure additionally wouthout affecting the flowrate.  
Once the flow is choked: the math changes and you use a different equation.  Any manufacturer's sizing software is going to address this and automatically shift to the compressible flow routine. The compressible flow routine has 15 variables.  It is not fun.   If you are sizing valves by pencil, paper, and calculator you have too much free time.  

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