Ask the cheapest contractor to show you a few of his completed projects, choose one complex, medium and small work. Interview the owner if possible.
Ask how he do some of his contruction works, what is his personal skills.
As to how he carried out his contruction work, you may want to share with us or get some opinion in here whether is correct or not.
Ask about his current Work in progress job and visit the site.

I think sometime contractor charge cheaper because of demand and supply, and personnal know how (Skills), if they have to rely on sub contractor high chances will be expensive as he needs to make some profit too.

Like wise you may want to choose one of the expensive contractor to carry out this exercise.

Not sure whether it will help u to make a correct decision, but atleast it will help you make a better decision.

I am also going to renovate my house too. Wonder you could share some of your quoatation information for our reference.

Can you list out the job that you ask them to quote with measurement, because majority of them use RM per sq ft.

Commom one would be like:
Kitchen RC work top
Tiling
brick wall rendering
Demolish brick wall
floor concete slap
Kitchen cabinet
wall cabinet
walldrobe
wiring
plumbing
etc