So first you give your Physical constraints in UCF, do Placement and Routing. Then Generate Post Place-Route Model from ISE. It will be a VHDL or VerilogHDL file (as you require). Then write your test bench either by yourself or by waveform editor. While doing that your stimulis should be such a way that you check the performace in worst case scenarios and whether the given constraints are met or not.
Then add this Post place and route model with test bench to Modelsim project. Also add the Unisim, Vsim libraries whose HDL sources you can find in you ISE Installation folder. Choose those libraries for the Device for which you placed and routed. Compile those libraries first in Modelsim and then Compile the whole project. Your Test bench will be the top level module. Now you can run it and get the complete simulation result