This strength or weaknesses could be as result of short term demand and supply but can also be caused by economic announcement results.
Ideally one should always trade the strongest currency against the weakest currency.
Use the Forex Trading currency tools below to select a timeframe and currencies to view
Other sources of relative strength of currency information
FX Street provides heatmaps of currencies.
Use this link or click on the picture for more information: http://www.fxstreet.com/technical-studies/currencies/heat-map/
When placing a trade in the Forex market one should realise that you are actually doing 2 transactions at the same time. Buying / selling one currency and selling / buying the other. Then when you exit you am always doing 2 transactions again selling / buying one currency and buying / selling the other. So when you sell the EURUSD for instance I am actually selling the EUR and buying the USD at the same time.
If you are looking for reasonable moves in the market then focus on the very strong currencies and on the weakest currencies. Always always try to BUY the stronger and SELL the weaker currencies. That way you get the economic fundamentals working in your favour and you know you are always trading with in the direction of a strong trend. The market is basically telling me the way it wants to be traded using this approach.