As a somewhat new person to airsoft(about a year in) this is something that really requires more effort than most people put into this. I know some lax fields will just allow you to bring any guns with a 400fps limit but wont check or do poor checks so people think that they do not have to find out their fps. There are many guns that will be hot for cqb environments right out of the box, and in the case of gas guns, can potentially be well above outdoor limits. Just because you werent checked doesnt give you an excuse when you do some serious damage to someone.
Gas guns not only have different charecteristics with different bb weights, but they also can be greatly affected by weather and the amount of gas you have in your resevoir. Ensure that when you chrono that you have a full fill of gas to ensure that your maximum possible fps is recorded. If the temperature heats up, your fps may also increase. If you go play on a cold morning and you are just below fps limits you may go over once it starts to warm up.
Its your responsibility to ensure you are not hurting someone, you can only have fun if everyone is safe.
just a horror story from my paintball experience that ensured I picked up a good airsoft chronograph(that can also handle different weights). I was at a field and a kid who didnt know his gun well showed up. Took a shot at someone and it punched through BOTH sides of his hopper(hard plastic filled with paintballs, it would have broken a clear airsoft gun easy) because he screwed with his gun and didnt care how it affected others. We chrono'd it and the average was about 615 fps....