海关人员究竟怎么判断你从JB入境需不需要付GST的？

第一、只有新加坡公民和永久居民才能有一定量的免GST
其次，如果当天进出的话，能免GST的商品总价不能超过S$150 （海关人员还是挺聪明的。比如奶粉来讲的话，某海关人员透露，能携带的重量大概是5公斤，大概就是3罐大罐的。算一下，还真的差不多。所以一般情况下，如果扫描出来有四个桶状物品，基本上他们都要拦下来检查的。当然看他们心情，多出一点不一定算你GST就是了。）
具体条规看下面。

Goods and Services Tax (GST) Relief All goods brought into Singapore – including new items, souvenirs, gifts or food products – are subject to 7 per cent GST.
However, travellers can enjoy GST relief on goods meant for their personal use depending on the time spent away from Singapore:
Time Spent Away from Singapore Value of Goods Granted GST Relief 48 hours or more S$600 Less than 48 hours S$150 Travellers are required to pay GST only on the value of goods that exceeds their GST relief and on goods carried for or on behalf of other persons. GST is levied on the value of the goods, which may include the cost, insurance and freight (CIF) plus other chargeable costs and the duty payable (if applicable).
Travellers should declare and pay the GST for their goods at the Customs Tax Payment Office.
There is no GST relief for liquor, tobacco products, petroleum, and goods imported for commercial purposes.
Holders of a work permit, employment pass, student pass, dependent pass or long-term pass issued by the Singapore Government and crew members are not eligible for GST relief.
Reminder It is the responsibility of travellers to make an accurate and complete declaration of all goods exceeding their duty-free concession and GST relief in their possession, including goods carried for or on behalf of other persons.
Those who fail to declare at the Red Channel can be fined and prosecuted in court. More information on customs offences　here.