If you feel drafts they need weatherstripping. You also may use a smoking incense stick to find drafts. If the smoke dances, you have a place to seal. Keep the burning tip away from all flammable objects. Seal leaks around electric switches and outlets.
Gaskets are available that fit behind the switch plates and outlets to prevent drafts.
Don't underestimate this one. Install sweeps or shoes to stop air from sneaking in under outside doors. If the crack under your door is too wide for store-bought weatherstripping, make a cloth snake, fill it with sand or beans and lay it against the bottom of the door to block the air.