We don't know for certain, other than to look at their last budget.
One saving they were going to make was in freezing the income tax threshold, so as to keep those on lower incomes paying more tax. They did increase an already massive tax break for the super-rich though, so savings there would have been largely, if not completely wiped out.
It's not in labour's interests to tell people where they would have cut. When labour were in power, Alistair Darling did make a vague mention that they would have to make worse cuts than Thatcher did in the 80s, although I don't ever remember seeing any details of how - and understandably so.
Far better for the labour party to sit back and pretend the debts could be paid without raising taxes or cutting spending - let the coalition take the heat for the difficult times ahead, then fight to regain power once the economy is doing better.
I'm not sure I'd be willing to believe Ed Miliband though - even if he told us. Comparing his rhetoric to his voting record suggests a lack of honesty.
Before becoming labour leader, he was Secretary of State for Energy and Climate Change.
His rhetoric on climate change:
"Every*thing we know about life is that we should obey the precautionary principle"
"climate change is the greatest global threat facing our generation [...] it should be at the very heart of our plan for a successful economy, at the centre of our foreign policy and integral to our mission to change Britain "
"greater certainty about emissions is necessary to provide the strongest incentive to business"
His voting record:
http://www.publicwhip.org.uk/mp.php?mpid=40208&dmp=1030
Note in particular, this one:
http://www.publicwhip.org.uk/division.php?date=2008-10-28&number=296&mpn=Edward_Miliband&mpc=Doncaster_North&house=commons
and remember the quote, "greater certainty about emissions is necessary to provide the strongest incentive to business".
He has also made political capital out of his opposition to the war in Iraq. Here is his voting record on that issue:
http://www.publicwhip.org.uk/mp.php?mpid=40208&dmp=975
I'd expect a similar level of honesty wrt tax rises/spending cuts.