When a strain occurs you can recognize them through the fact that they:

will occur in the muscle belly – if you feel it in the soft tissue it is most likely a strain
is a sudden sharp onset pain
there is pain on lengthening the muscle as well as contracting it
if the strain is severe enough swelling or bruising may appear
if the strain is a tear, a divot or gap may appear in the muscle or it may rend apart altogether
Strains are graded on a 1-3 scale.

Grade 1 tears consist of minor tearing of the muscle. There is no bruising and little to no swelling, but the pain is present in the soft tissue. The amount of pain is often variable to how the person perceives it. It is possible that there is no pain on concentric movements and only eccentrics, and there is unlikely to be pain with some light pressure to the area. This is what most often occurs with people who workout.

Grade 2 tears are partial tears of the muscle. There is likely to be some swelling. Bruising is still variable but most likely will occur as the tissues are damaged/ruptured enough that there will be blood leaking out. Both concentric and eccentric movements will hurt, and putting pressure on the area will cause pain. Since the muscle has a large injury it will result in limited range of motion.

Grade 3 tears are complete ruptures of muscle or close to it. There will be swelling and bruising. There is also likely to be a divot/gap left by the muscle where it tore in two. It is important that if this occurs to throw RICE it right away – rest, ice, compression, and elevation – and to get to the emergency room ASAP to get it checked out

I would also advise going to the ER in the case of a grade 2 partial tear as well, but it may not be necessary depending on the circumstances. In any case, this article is mostly directed at grade 1 and low grade 2 strain prehabilitation and rehabilitation.