Before deciding the liquidity of a bond or its credit spread, investors need a bond trading brokerage account. tradeMONSTER is a fairly sophisticated trading platform that enables traders to trade around 30,000 bonds.
The best way to trade bonds is to do it like bond traders do sans the advanced equipment of course: juggle bond price watching with studying the market for any news that will affect it.
There arbitrage opportunities that come about when an astute trade identifies a bond with a higher yield than usual or a similar scenario. I guess the easy thing to do is to look at what Morgan Stanley, UBS, Credit Suisse, BNP Paribas, etc are doing with their bond trades. In addition, look at smaller shops too such as the Goshen, Indiana based Everence Financial and foreign banks like the French Credit Agricole Securities.
As always, when I touch on these 'How to' topics, I never forget to leave a certain something for people aspiring to pursue it as a career to peruse. Being a bond trader, enables you to be well compensated in a hard to enter field which requires a quantitative background and a prestigious degree from a top ranked university.
Also, get on LinkedIn and Mashable and see what sort of skills (software, technical, etc) it requires to have a successful bond trading career.