In my experience, not very, so assume they have them and use protection.
Not because they are selfish, don't care, or think it'll stop their chances of knocking boots, but simply because they, like gals, don't get checked out until symptoms show up. Some STDs (HPV for example) are usually symptomless in guys, just like some (gonorrhorea) are usually symptomless in gals.
Don't let it stop you butterflying as much as you like, just be sensible and carry condoms with you. I am in favor of suggesting the dual HPV vaccine for young women as well, as condoms are not perfect protection against viruses, but that is your call.
college guys, older guys, middle age guys, nobody is exempt from not being forthcoming. MM is right, take control here, it's YOUR health, and until you are married to someone who has actually vowed to be with you in sickness AND...you just can't count on anybody else to protect you from what's out there. your mama maybe, but all she will say is...just dont have sex then and that's the only way to be sure lol