Water tests during travelling

mgb addresses just one tool: filtering pump. According to the source here, there are two other ways:

If sources of bulk, safe drinking water are not available three better choices are; a filtering pumps, an ultraviolet light purifiers (search: SteriPen) or an oxidant purifiers (search MSR MIOX). These do have a little bulk and require an initial investment, but they can payoff rapidly. (If you process ten liters of water a day, at a value of one dollar per liter, in ten days you have recouped $100 dollars of your investment.) Filtering pumps tend to be the most versatile and labor intensive, including requiring the most maintenance. There are several brands of differing quality available, priced from $30 to $200. People have had the most problems with models that don’t have filters that can be completely cleaned in the field. The brand that seems to be the most durable, problem-free is Katadin, but it is also one of the slowest filtering pumps and the most expensive. The last two, hi-tech methods, require clear water, which is increasingly available, and use batteries -- which can be rechargeable. The MIOX also need salt and adds a little taste to the water, which is mostly an issue if you start with a high mineral water that already has a taste. The UV pen is the most fragile, but generally can be sufficiently protected. The UV pen is generally the fastest. For more discussion on water treatment see http://www.rei.com/learn/Camping/rei/learn/camp/clwatertreatf and read and click through the pages.

But it is not actually totally true, there are even more ways: summary diagram about different purifying methods here (sorry pasting it tweaks the layout). Please, note that boiling one minute water kills protozoa, bacteria and viruses like the expensive purification methods such as purifier, filter, UV light, mixed oxidant, chlorine dioxide and iodine. More on the "effectiveness" diagram here. Some excellent tips/analysis/comparison with mgb's Katadyn suggestion, boiling and other purifying methods here.

Comment by the user Martin Beckett to the original answer:

The steripens are pretty good but: They kill bugs but don't filter dirt, and aren't effective in cloudy water - so are best if you have a supply of clean but suspect water (like a tap). If you are purifying water with pens/iodine etc then you have to be much more careful about which containers you use for dirty/purified and how long it has been treated for. The equipped.org link has more info about harder to kill things like giardia.

As far as I know, there is no practical way to test the safety of water before drinking it.

For example, one common pathogen in water is the parasite Giardia. According to this link, the only known way to test for it is to filter several hundred gallons of water and examine the residue with a microscope (and know what the cysts or eggs look like). Even if the cysts are not observed, this is not conclusive evidence that the water is free of this parasite.

Testing for bacteria would probably require culturing a sample for several days. Viruses would be even harder.

So unless you carry a complete laboratory along with you, and are willing to spend a week testing each water source before drinking, testing will not help you. You will just have to purify your water every time, and hhh's answer has good pointers on this.