If the dart sticks solid into your bicep, it's into muscle. If the dart isn't even slightly anchored, and is very wobbly, your in fat. A way to test is to let go of the syringe after you insert it, if it stay standing up yer good, if it flops down and bends the needle, repin.
A good way to keep from pinning into fat is to use a longer pin and put more pressure on the syringe(not the plunger) until your sure its intramuscular:star-wa:s