There are a couple ways. If you have timing marks on your model which show TDC for the #1 cylinder, you can start there. If you know the firing order, you can do the math and turn the engine the appropriate amount so the next cylinder is tdc, e.g. 4 cylinder, turn 90 degrees to get to the next.
Another way; when the valve cover is off, crank the engine by hand with a bar or whatever and watch the valves. Watch for the intake valve on a cylinder to go down, then back up. You know that piston is headed back up and you can estimate that another 20 degrees or so of turning will get it pretty close to tdc.
In reality though, you're normally going to have to pull the injectors or the glow plugs and you can just stick a straw down the hole and feel the piston as it comes up to TDC.