The DUP leader has warned she will not support any Brexit deal that could lead to new economic barriers between Northern Ireland and the rest of the UK.
Arlene Foster was speaking after a meeting with the EU’s chief Brexit negotiator Michel Barnier in Brussels.
The government is expected to give details of its border backstop plan in the coming days.
Mrs Foster said it would be studied carefully.
“What we need to see is the legal text,” she said.
“We cannot talk in a vacuum. We need to see what has been proposed and we will check that against what we have called our red line.”
Mrs Foster said she hadn’t been shown a text of a “hybrid backstop” plan being worked up by UK officials.
She also clarified her controversial remarks about the party having blood red lines.
“What I meant is deep red, they aren’t pink,” she said.
The backstop arrangement could apply if the Irish border cannot be kept as frictionless as it is now in the context of a wider deal.
Analysis: BBC News NI’s Gareth Gordon in Brussels
The large number of journalists waiting to hear Arlene Foster’s take on her meeting with Michel Barnier tells you everything about the DUP’s meteoric rise up the political pecking order.
Small regional parties do not normally attract the attention of the international media.
However, the DUP is the party which Theresa May relies on to stay in power, and the party can – theoretically at least – vote down a backstop deal.
There was general astonishment in the media centre when Mrs Foster said she hadn’t yet seen the UK’s so-called hybrid backstop plan.
If it does contain provision for light-touch regulatory checks on goods between Northern Ireland and Great Britain, she left no doubt that would be unacceptable to the DUP.
Mrs Foster added that she would wait to see the text before commenting in detail on the issue.
When she does, the DUP might have a tough decision to make, and so might the Prime Minister.
Representatives of Northern Ireland’s two main unionist parties – the DUP and the Ulster Unionists – have been meeting Mr Barnier, to discuss Brexit.
Ulster Unionist leader Robin Swann was guardedly optimistic about the possibility of “a good deal for the UK which involves NI and the EU”.
“The feeling we’re getting coming from the negotiators, the ambassadors and from part of the negotiating team that we met seem to be indicating we are going towards a deal,” he said following his party’s talks with Mr Barnier.
“We are seven to ten days from this deal having to be on the table.
“I don’t want to be overly optimistic or to spook the horses but I think serious negotiations are going on.”
Mr Barnier recently called for “less drama” over the so-called Brexit backstop regarding the Irish border.
The EU negotiator held talks with four of NI’s other parties last Friday.
Mrs Foster had earlier warned that the party would not support a Brexit deal that could lead to any new checks on goods coming into NI from elsewhere in the UK.
Speaking ahead of Tuesday’s meeting, she said: “The United Kingdom single market must be protected with no new borders between Northern Ireland and Great Britain being created. From day one this has been the DUP’s only red line.
“Over 70% of all goods leaving Belfast port are destined for Great Britain. To create a barrier to that trade would be catastrophic.”
Following their meeting with Mr Barnier last week, Sinn Féin and the SDLP said they rejected the possibility of Stormont having a say over what kind of backstop may emerge.
Proposals being worked on by the British government are said to include some kind of role for MLAs, but the nationalist parties are fearful that this would give rise to a veto for the DUP.
On Saturday, EU officials expressed optimism that a Brexit deal could be struck by the end of the year.
But Irish taoiseach (prime minister) Leo Varadkar said there is still “a fair bit of work to be done”.
NI secretary Karen Bradley told the Conservative Party conference the government would “never ever allow a border down the Irish Sea”.
The UK is due to leave the EU on 29 March 2019.