Ask your friends! As hot as that sounds to me, I know it’s impractical. If you search through posts tagged advice you can find a couple of great posts about squirting (You will also find those posts under the tag squirting, but with more porn!).
Sorry for not linking here but I am answering mobile and it is a hassle.
I’m going to go on a bit of a tangent here because I wan t to address all areas of this, but I promise I will get to your specific question soon.
As with any sex, if you are unsure of your partners STI status you need to use protection. Either in the form of an actual dental damn or in the form of an improvised one. To improvise a dental damn, cut the tip off a condom (preferably non-lubricated, that shit tastes nasty) and cut up one side, unroll and ta-da! You can also use plastic wrap from your kitchen. That being said, you are all adults and I expect you know how to behave as such.
Now, on to your actual question. Aside from STI’s there is the slight chance of contracting E. coli from unprotected anilingus. That chance is pretty slim, and can primarily be avoided by good hygiene. The squick factor for feces is pretty high, and I do understand that. Keep in mind that the human body doesn’t typically store fecal matter right inside the first sphincter.
I highly recommend you and your partner(s) discuss anal play before you just jump right in and shove your tongue up their ass. Yes, it can be hot for everyone involved when something new and unplanned happens, but when exploring it is probably better to communicate what you want and don’t want and what both of your concerns are.