This is basically a copy of the bounce checking/angle part of nolem's
Bounce Tools, but in javascript so you can use it outside TF2.
https://www.dropbox.com/s/23ua2kmgy3updn7/boucechecker.html?dl=1Enter a height and optionally the angle to start looking for a bounce from (defaults to 60), select options from the drop down depending on what bounce type you want and it will show you the angle (and range).
You can also check "special starts" (couldn't think of a better name) which involve walk right/left/forward/etc, looking straight down and firing. Some of these are more reliable than others.
If you want to change which options are enabled by default, open the file in a text editor (like notepad) and edit lines 14-34.
Should hopefully work in all browsers (tested in Firefox, IE and steam).
If you find a bounce that doesn't seem to work, let me know and I'll check it out.
Code in a more readable format if anyone is interested
https://www.dropbox.com/s/lzg0adt5g26nay3/TF2Bounce_source_119.rar?dl=11.1.9 - Added Cow Mangler checking to special starts (can't check angles for cow mangler)
1.1.7 - Enabled special checking by default and added a small set of config options at the top
1.1.6 - Fixed typo/grammar
1.1.5 - Slightly changed values for checking bounces
1.1.4 - Fixed small issue with showing starts when no angle was found. (Thanks to Xtra)