If you search your route on GoogleFlights and pick the flights you want, you'll see a box at the bottom center of the screen below your chosen itinerary that shows Google's assessment that prices are low, typical, or high. To the extent you trust that, it's a tool you may consider.
Kayak does something similar with a blue box at top left of the results screen--but I've often seen them say they don't have enough data for the route chosen. Same caveat as above: useful if you trust it.
You're best served to decide how much you're willing to spend and buy if the price is acceptable to you. Others have posted about general timeframes to use for domestic/international and peak/off-peak dates.
You're profile shows you in Glasgow (Scotland, I presume); I do not know if GoogleFlights and Kayak search algorithms will work differently for you, but they may.
Safe travels!
Last edited by AAir_head; Feb 28, 2020 at 12:45 pm