Limiting Magnitude Charts
The charts are in PDF Format inside a ZIP bundle for ease of download.
They are very easy to use. Take the chart outside, find the area the chart covers, there are 5 charts:
Chart 1 - Orion
Chart 2 - Corvus
Chart 3 - Scorpius
Chart 4 - Eridanus (Achenar Area)
Chart 5 - Musca and Crux
On the chart, some stars are marked with their magnitudes, from around +3 down to about +7. Find the faintest star that you can see, and your limiting magnitude is about half way between that value, and the value of the next fainter star that you cant see. Simple.
The link is below. Just download, print them and have a go.