First of all, weightlifting and powerlifting use a common color code for their weight discs. Either the rim or the entire disc will be colored. Here is the color code:
COLOR WEIGHT(one disc) WEIGHT(pair) red 25kg (~55lb) 50kg (~110lb) blue 20kg (~44lb) 40kg (~88lb) yellow 15kg (~33lb) 30kg (~66lb) green 10kg (~22lb) 20kg (~44lb) white 5kg (~11lb) 10kg (~22lb) black 2.5kg (~5 1/2lb) 5kg (~11lb) chrome 1.25kg(~2 3/4lb) 2.5kg(~5 1/2lb)Plus, you have to add 25kg(~55lb) for the bar (20kg) and collars (2@2.5kgs each=5kg for the pair). So now, you can figure out about how much most lifters are attempting by just observing the bar. Plus, there are small rings that weigh 250 grams each. These are not noticeable from a photo, but they are only used for record attempts.
For example, say you see a bar with one red plate, one yellow plate, and one small black plate on each end. The total weight on the bar would be 50+30+5+25=110kg.
There is also standard procedures for loading the bar: you will use pairs of 25-kilo plates, until the weight to be added less than 50kg total. Only then do you add lighter discs. Thus, all of the interior discs (that you can't see the sides of) should be 25-kg discs.
Now, in smaller meets, they might not necessarily have the proper weights. Also powerlifing may use 45kg (~99kg) plates; especially for the squat and deadlift. In powerlifting, the 45-kg plates are often gold-colored. They don't use anything greater than 25kg in weightlifting anymore. They decided that it was difficult and injurious to the spotters to have to handle individual plates of this size.
If you're looking at old weightlifting photos, you might see an old green 50kg plate. For example, in this often-seen image of Vasily Alexeev breaking the WR in the jerk at the '76 olympics, you might notice that the two interior plates are thicker, green 50kg discs. (Pop quiz: how much is Alexeev lifting in this image? Answer at bottom of this page).
Recently, weightlifting introduced a women's bar. The diameter is 2mm smaller, and the sleeves are shorter. This bar weighs 15kg, so with the collars, it's 20kg, not 25. The length inside the collars is the same, but the bar has noticeably shorter sleeves (the thick part outside the collars on which the weight discs are loaded). This lighter bar is also used in youth competitions. Powerlifting does not have a lighter women's bar.
Also, a lot of small powerlifting meets in the U.S. will use imperial plates, and run the contest in pounds. The plates often used here are stacks of 45 lb plates, and sometimes 100-lb plates. The smaller plates are 35-, 25-, 10-, 5-, and 2 1/2 pounds. These are the increments in use in most American gyms.
Sometimes you see a photo of a lifter in training, and you wonder how much is on the bar. Well, if you know he's an American powerlifter or bodybuilder, chances are he's using imperial iron plates, which are typically 45 pounds each, and about one inch thick. Sometimes for magazine photo shots, they will add some fake plastic plates to make you think they're lifting more!
Weightlifters often train with bumper plates, which are thicker, but not always clearly colored on the rim. There are 15-, 20, and 25-kilo rubber discs, which are all about 5cm (2") thick. There are also full-size 10-kilo rubber discs, which are a little thinner, and sometimes old 50kg plates, which are much fatter. We have a pair of 50kg discs, but we only use them as a platform for box squats! There can also be fatter 25kg plates.
The procedure for loading the bar has been to put the first (inside) plate so that the number markings are facing the inside, and then put all additional plates on so that the numbers are facing the outside. This makes it easiest to see the weight of the plates. Most modern competition discs have numbers on both sides, so that they can be loaded with either side out.
Most lifters will do this in training also; however, most nonlifters (bodybuilders and gym rats) will load all the plates with the numbers facing in. I don't know why, it just seems that that is the habit engrained, perhaps, from high school.
So now, I hope you can figure out the approximate weight of what people are lifting by just looking at the bar.
Strength athletics
Answer to quiz: Alexeev is lifting 255kg in this photo. The bar and collar weigh 25kg, the green plates weigh 50kg each. There are also two red plates on each side, weighing 25kg each. In addition, there is one yellow 10kg and one yellow 5kg metal disc on each side.
http://www.geocities.com/aedziepak/strength/howmuch.htm