Yea, there's no indication similar to registering. You just have to trust based upon sight and feel that you're in a muscle. It's really not difficult. You'll find that most muscles in a relaxed state are easy to grab and chart using your sense of touch as a guide. It's very common to administer Intramuscular injections in the Deltoid, which is basically your shoulder. The way in which your Deltoid interacts with the other nearby muscles makes the process for locating a proper injection site pretty easy.
Think about a V shape. You're going to locate the upper end of your arm. This is going to be the clavicle. You should be able to find it easily. Now, take your hand and spread your two fingers into a "peace sign". Line up this V shape with the Deltoid muscle and you'll find that the shape pretty much forms a perfect boundary with the muscle itself. Now, you're going to want to inject at the "base" of this triangle, that is, not at the "point" of the triangle, but the part with a larger surface area closer to your clavicle.
Now, grab the muscle and really feel where it's at. Manipulate it, learn it and then grab your rig. You're going to want to insert it at a roughly 90 degree angle all the way down until the base of the barrel is flush with the skin. Then, you depress the plunger. It's normal to feel pain during the process, but it should be nothing more than 2 seconds of minor discomfort. It's really not bad. The Deltoid is considered the gold standard for IM injections and the glutes are a common second. In general, any injection is better above the waist than below due to simple gravity and shit.
