Sitting on couch and had a spider crawling on arm and it was very small but looked like brown recluse. How do ik if it bit me?

Severe reaction. It should be pretty obvious, because the bite causes a nasty reaction. The skin may turn black at and near the bite.
Brown recluse bite. Brown recluse spider bites often go unnoticed initially because they are usually painless bites. Occasionally, some minor burning that feels like a bee sting is noticed at the time of the bite. Symptoms usually develop two to eight hours after a bite and include itching at the site of the bite, nausea, vomiting, fever, and muscle pains. Immediate medical attention is recommended.