In the cockpit, the angle can be calculated from ground speed and VS. For a three degree descent, you want you VS to be 100-200 higher than 5 times ground speed. (Note: VNAV does 2 degree descent by default)
If you are using the HUD screen, the green circle is the FPV. This shows where your plane is going, so you can tell the descent angle from that. However, the more typical use of the FPV is to aim it at the start of the runway.
Another note: when taking off, many aircraft will pitch to around 15 degrees. This is the nose pitch, not the climb angle (otherwise you would be climbing at 4000fpm). This angle is indicated on the PFD in the cockpit, and by the long line with the dip in the middle on the HUD.