how are spreads calculated on backtests?

If I write an EA for EURUSD and get it to print (Ask-Bid) it just prints 2

How are the spreads decided in backtrades? is there just a fixed number decided by the broker?

it just grabs the current Bid/Ask and uses it through the entire test.

not exactly... your historical data only stores bid prices. Ask is automatically calculated using bid + spread.

To know what spread is being used:
In your tester panel, select a symbol and click on the "Symbol Properties" button. you'll see what the spread is. That's what the tester is going to use throughout the whole of your testing period.

regards,
Zen