You could use a ***, i.e., a frame alignment guage from Park Tools. Another quick, easy and cheap thing I've done is tie a thread in one dropout, pull it to around the headtube, then pull it around to the other dropout, then tie it. Now, measure the distance between the thread and the seat tube on both sides and compare.
If you are using a wheel to check straightness of the rear end of the bike, you have to make sure that the wheel is absolutely true and dished properly! Also, tires don't always seat exactly right, so be aware.