There's seldom a 100% way to be sure it's your time to speak. I'm also a straight cis white guy. I usually take the stance that I'll say something if no one else is speaking up because maybe the marginalized people don't feel safe speaking on the issue. If someone else has spoken who's closer to the issue I'll just back them up. If it's something I'm less sure of I'll say "that sounds kind of (whatever style of reactionary), probably don't say it." and back down if I get pushback from more affected groups.
There's seldom a 100% way to be sure it's your time to speak. I'm also a straight cis white guy. I usually take the stance that I'll say something if no one else is speaking up because maybe the marginalized people don't feel safe speaking on the issue. If someone else has spoken who's closer to the issue I'll just back them up. If it's something I'm less sure of I'll say "that sounds kind of (whatever style of reactionary), probably don't say it." and back down if I get pushback from more affected groups.