What you should be looking for
Is your company dispatching you as ITAKU (委託: dispatcher/employer is responsible for your work at the site) or HAKEN (派遣: both the dispatcher and the dispatch receiver can be responsible for your work at the site)?
These changes apply only to those working in a HAKEN situation.
In the past HAKEN was limited to three years (for all HAKEN not just the employee) so companies used ITAKU to carry on dispatch forever.
Furthermore, if you are in a HAKEN situation, the dispatch receiver is also responsible for some aspects of your working conditions.
How long have you been dispatched to the site?
In principal, dispatch is limited to three years per dispatched employee; but (for example) if your position or site is changed, the dispatch can go on longer.
As of 2018, will you have an unlimited term contract or not?
If you have an unlimited term contract with the dispatcher, the three-year limit at any given site does not apply.
Are there other directly hired employees working for the dispatch receiver doing the same job as you?
If there are, you need to try to find out about any differences in your working conditions because the employer is not allowed to offer different working conditions to those who are directly hired versus dispatched employees.