Assessing horizontal viz with your 'eyes' (in my book) may not be easy to a new-ish pilot?
One trick that was taught in the mil on occasions was to use timing...5000 metres is roughly 3 miles...which at (say, for example) 90 knots ground speed is 2 minutes flying time. So if a ground feature on track pops into view and you overfly it in less than 2 min the vis is less than 5 km.
Adjust figures above as appropriate for your type and speeds, and yes, it is only a rough and ready method.