Very hard to do when just lifting up the rear wheel...the bike ideally needs to be upside down, with the handlebar resting on an open recycling box to protect the gear on the bar. Turn off the power.
Once acheived, rotate the wheel, and brace a pencil against a stay so the tip is right against the metal wheel. Rotate and look for a wobble. With patience, and reading or viewing truing instructions, a wheel can be straightened out, keeping in mind that spokes must be worked on in groups, not individually. It's a challenge, but doable. Let a bike shop do it, and watch.
First, though, feel each spoke and look for obviously very loose ones...these can be gently tightened to the level of their neighbors...their neighbors are NOT the next spoke, but the one after that.
Good luck, and remember deep breaths, and go slowly.