Careful with the capacitors....
While most larger types you'll find will be just like Tinhorn described, there is another type - the tantalum electrolytic - that is marked exactly opposite! Usually an arrow pointing to a " + " mark. This marked lead would go to the more positive side (pin 6&7 of the 555 timer)
Another way to tell the polarity on radial type caps (both leads coming off the same end) it that the positive lead is always longer. A lot of smaller capacitors, especially the tantalums have values which are virtually unreadable. Go by the lead length and you will be OK. If you hook a tant cap up wrong, you better duck quick and keep a clean pair of underwear nearby because its pretty impressive when it blows.