@escknx have a look here: https://wiki.e3d-online.com/E3D-v6_Troubleshooting
"An easy way to check that your heatsink has sufficient air flow is to simply feel the heatsink with a finger after a print has been running for some time. (Be careful not to touch the heater-block, it will burn you instantly) The entirety of the heatsink should be cool to the touch, including the bottom fins closest to the hot parts."
But take care! This text's for the e3d v6 heatbreak. Might differ quite a bit from mosquito.
My v6 heatbreak is easily touchable. Only the lowest fin - directly above the heatblock - becomes warm. With my duct it's practically impossible to cool the lowest fin from below - but it doesn't affect the print. When i decreased the voltage of the fan below 20V, the fins became feelable warmer to the touch.
Be aware, that using an infrared thermometer might give wrong readings as it probably measures the temps in the middle of the heatbreak - not at the top of the fins. In that regard the finger-test - carefully - is more reliable.
Your shroud might fit better to the 40mm fan. If it properly directs the airflow to the middle of the heatsink, everything might be fine. With my delta's comparably small hotend there's not much space to mount something airflow-optimized. And the 40mm fans diameter versus the heatbreak diameter is quite different - which creates quite a bit of an airflow-obstacle working against the fan.
https://www.thingiverse.com/thing:2084339 With that highly optimized (filament-cooling) duct one can feel, that almost as much air is coming out of the duct as when the hand is held directly in front of the fan. Take other ducts and almost no air's coming out.