Below the Kick start there should be a 'Sight Glass'. The oil level should be to the Top of the glass. If you can't see anything, use Spirit Cleaner to clean the Glass then have someone keep the bike level. Using a strong light source, check the level.
Step 2, using a clean container, dump the oil from the engine keeping the bike level.
once drained, measure the amount collected. Most XT 250's hold 1.3l