There are a number of sites that allow you to submit your missions for beta testing or offer mission beta testing forums. The most popular links for beta missions are:
All of these sites have strict rules about beta submissions. Make sure that you adhere to them so that you don't waste their time and yours.
I highly suggest that you always include a ReadMe text file with your beta missions, documenting the minimum OFP version required and any required addons. Mentioning your email address is also a good idea, if you want to invite people to contact you privately regarding their beta testing of your mission.
One logistical problem that may be hard to resolve with beta missions is that your beta mission file(s) may exceed the maximum size allowed for submission. Here are a few suggestions to overcome such limitations: