Everyone’s bodies are different and each person has different concerns, so it can be hard to answer this objectively for everyone. Most people want to enjoy clean anal without a mess, but there are different ways of achieving this. In most cases, with a balanced diet that has enough dietary fiber and not too much fat, simply using the bathroom beforehand is enough. The rectum, where sex generally takes place, doesn’t tend to remain full except for right when you feel the need to go to the toilet. Inserting a plug or dildo right before sex can be an easy way to quickly check and make sure, too.
If you want to really ensure cleanliness more, you can clean out before having sex. But this doesn’t need to be a big ordeal—certainly not a full deep enema every time, just use a small enema bulb/anal douche with a small amount of warm water to flush out the rectum several times in a row. That’s it, unless you’re doing big toy/depth play, you don’t need to do more than that, and using too much water and getting it deeper into the colon past the rectum usually ends up making things messier (unless you commit to a full, deep enema) rather than being helpful.
With experience, you can usually just go to the bathroom first and make sure or be able to tell by listening to your body whether you’re likely clean and ready for sex. With a regular digestive routine, you can usually tell when is and isn’t a good time. Some people choose to use a light enema/douching right after going to the bathroom each day so they’re ready for the rest of the day assuming a regular routine.